1.
Yes, Co-Star stabilizers on the Filco will definitely pose a problem for custom keycap experimentation, making Leopold the better choice for you.
2.
As for custom switch longevity:
The only custom switch I have personal experience with over a 2+ year period at this time are SP Star Meteor Orange [Cherry-like light-tactile] in a soldered build.
Interestingly, Leopold thought that SP Star Purple and Meteor Orange were reliable enough to solder into their factory keyboards.
I also have 2 keyboards soldered with hand-built Ergo Clears that have been in operation for 2+ years. It appears to be a reliable modification.
Both Cherry Ergo Clear and Cherry MX Purple will give you options for Cherry medium-tactiles. The official Cherry Ergo Clear at the most established of these, I think. And the new Nixies are definitely an option. [You might want to try some nice TX springs in them, maybe about 65 G 14mm (S) or regular 63.5 G Spirit.]
There is also something called Cherry MX Orange, which I think is a medium-weight linear.*
3.
As for custom keyboards, I can only speak from personal experience. I enjoyed the
NCR-80 affordable custom for its Cherry-inspired design and its lightness. It happens to have a good typing angle for some. It feels like plastic and sounds like a hollow milk carton, but it is supposed to. As the Cherry boards did.
I have been using soldered NCR-80s for 2+ years, and they have held up, despite being meant to be cheap. You may have to make minor adjustments to it [fit and finish] to get them to close up nicely or have the legs be even.
I personally think that the NCR-80 has good typing angles, at least for where I place them. YMMV.
The flaw of the NCR-80 is that it is either plateless, or uses CF / FR4 plates. The former is awkward for tactiles, I think, but that is subjective. The plates aren't anything special. The NCR-80 doesn't use a sophisticated mounting set-up, and I prefer gasket or especially top-mount.
Also, the hotswap variant has a north-facing PCB, which may cause interference with Cherry-profile on R3. [Not everybody notices this interference.] I believe that soldered PCBs are south-facing, which is the correct alignment.
It is very easy to move an NCR-80 around your workspace, and many people have noted its handiness in that regard.
Variants of the NCR-80 are available in ISO.
The
KBD8X MKII from KBDFans [designed by AI03] was known to be a very reliable keyboard that could handle multiple de-soldering/re-soldering operations. I don't know what the MKIII is like, but if it is like the MKII, then it is very reliable.
I think the KBD8X MKII is comfortable, and has good typing characteristics. [It is top-mount, IIRC]. The only flaw is that there are no adjustable feet. So you may have to use risers if the typing angle doesn't work for you. This is the case with many keyboards.
Another issue with the KBD8X is that it uses a heavy weight at bottom. So I think it is meant to stay in one place.
So that's from personal experience. The NCR-80s and KBD8X MKII lasted 2+ years so far.
Now, if I were to speak outside of personal experience, there are other recommendations I could give. You want something that is entry-level and simple. Not too much of an investment, but known to be reliable.
My first recommendation would be a
Keychron V or Q series in TKL or 100%. I think they come fully-assembled [even the barebones]. You can do things to improve them, but they should be almost ready out-of-box.
They are meant for people entering the waters of custom keyboards. Ready-to-go, but modifiable. You can order extra mod parts, I think. You can adjust dampening, improve stabilizers, things like that.
They are hotswap. So maybe somewhere down the road it will fail. However, the reason I am recommending it is that people have been buying Keychrons for entry-level customs for more than a year now. And although I am sure some of them fail, Keychron is a real company. They should be able to eventually fix / mail a replacement if you order from your national distributor.
They are regularly stocked, and many people use them. So we have a pretty good sample size and timeline here. And it's not a huge catastrophe if your Keychron V6 fails 3 years down the road, for example, at least compared with more expensive decisions.
Keychrons are on sale right now at some retailers.
So there are affordable TKLs and 100% keyboards that have some track record of reliability and vendor service. [Although, still, dealing with Keychron and similar are worse than dealing with a very local storefront.]
If worst comes to worst, at least you can salvage your stabilizers, keycaps, and switches from a broken keyboard. So you don't lose your whole investment.
4. Now let's talk about the other option you were planning. De-soldering, and then re-soldering a Leopold.
It's very do-able.
But it requires skill. Leopold in particular at least used-to solder the switches in pretty robustly. So you have to have a steady and consistent hand to desolder. Then you have to carefully solder the new ones in, having already taken the time to improve the stabilizers while the switches were desoldered.
I have two Leopolds that have been adjusted in this fashion. See here:
https://geekhack.org/index.php?topic=103412.0
I should say, like Filco, there will be a lot of ping with stock Cherry tactiles in one of these cases. I think the cases amplify or encourage ping somewhat. They use metal plates, I believe.
You can reduce ping by hand-lubing the switches. The springs can be 'bag-lubed' using special spring-lube. It is very important to lube the springs. Even if you are using stock Cherry switches, for reliability, they still benefit from at least the springs being lubed.
Adjusting the dampening inside a Leopold may help with the ping somewhat. Something you can do while it is disassembled for soldering. You can also foam the spacebar. Maybe dampen the stabilizers with pads, somehow.
Unlike the Keychron, the Leopold are not designed for modding. So there are no aftermarket parts. I suppose you could replace the stabilizers with other plate-mount stabilizers, but the included ones were decent by 2018-standards. There are also no additional dampening materials or plates available, unlike with Keychron. But you can dampen a Leopold by other means.
Anyway, resoldering improved Cherry switches into a Leopold is certainly an option, and should be reliable. Will require considerably more work and skill than final assembly on a Keychron, I wager.
*For custom switches, you might also consider the Gateron G Pro 3.0 series. They are a standard, common mainstream choice. The G Pro 3.0 Yellow are highly-rated medium-weight linears. The G Pro Brown are a little more tactile than MX Brown.
They are affordable, and can become very cheap during sales. The G Pro Brown 3.0 don't benefit much from hand-lubing, as they are factory pre-lubed. I think that the similar Jupiter Browns may have slightly better factory-lube / materials. The Jupiter Banana [top-bump tactile] are also extremely common and highly-rated.
Some other linears that people seem to use without problems: Gateron Ink Black, Gateron Oil Kings [no work needed on those], WS Yellow, KTT Strawberry, AKKO Cream Yellow Pro.
In tactiles: Durock Anubis, Gateron Baby Kangaroo, AKKO Lavender Purple Pro, Gateron Root Beer float and the budget Everfree Grayish, Durock Medium Tactile. I haven't heard complaints about [hand-lubed] KTT Matcha.
The U4T also seem to improve with factory revisions. In theory, the Zealio V1R [smooth Ergo Clear] should be reliable. Currently on-sale. I think Varmilo once soldered the Zealio V2 into their enthusiast factory-keyboards.
If you find deals today, maybe pick up sample packs of 1-10x switches [that is to say, up to ten switches], to test later in hotswap. Or find a cheap $5 acrylic switch-testing holder somewhere. Save you tons of money in the long-run.