Hi Maledicted,
I started with typical gaming keyboards like Corsair, Ducky and a IQUNIX F96. A few years ago I found videos from Taeha Types and started playing around with some basic keysets, now I want to take things a bit more serious and I want to start soldering, I have previous background in electronics and PCB design but never on the keyboard space 
I still use quite a few Corsair boards. I love the aesthetics of the old K65s and K70s myself. You've done electrics and PCB design, but no soldering? That's interesting, I'm the opposite.
Switches that I tried so far MX Cherry Red, MX Cherry Brown and Gateron Red. I still don't have a very strong preference, I wanted to buy one of those "switches testers" to play around with other switches.
What's your preference here? Any advice?
I usually recommend
this 130 switch tester. It doesn't have most of the boutique switches, but it has a good smattering of everything else for a good price. A lot of the switches in the tester are basically duplicates (variations in mounting type and lighting support), so if you ever wanted to add other switches instead, you could.
I like a
lot of different switches, mostly vintage ones and out of those mostly vintage clickies. I'm not a fan of MX in general, so I haven't really tried many of the boutique MX linear and tactile clones. Of the MX compatibles, my favorites are the Kailh box clicky switches. I particularly like the jade, navy and pink variants. Very crisp and refined, as smooth as a clicky would ever need to be, and the jades and pinks are relatively low pitched, which is pretty uncommon for clicky switches. Out of all modern mass market switches made, I like Matias' clicky switches just a bit more, but those are incompatible with MX caps and boards. Overall, I think IBM's capacitive buckling spring switches, IBM's beam spring switches, and Alps SKCM blues are some of the best clicky switches ever made.
As far as advice goes, I would try as many switches as you can as cheaply as you can before you start making potentially expensive investments in entire boards. It is a good idea to play with everything on a tester for a while and figure out what switch types you seem to really like between linear, tactile and clicky and then start narrowing down what you like about those types. It is also a good idea to get a cheap hot swap board (that doesn't have Outemu hot swap sockets) for testing the switches you do like without committing to soldering them into a board. Tactiles are weird in that they're usually a plasticy and rounded feel (in the MX space) but there are some tactiles that are relatively sharp/crisp, like Alps and Matias.
Of the 3 you have tried, I think Gateron reds are pretty nice. Personally, I think that MX browns are the worst tactile switch still in production (very scratchy), but it has its fans. These "linear" Matias switches have more tactility than MX brown though.
I just ordered 2 new keycap sets: Infinikey Monochrome and GMK Striker 2 (+Salvun Classic Football). I personally love the profile of SA keycaps, there is something nostalgic about their shape - I'm negotiating with a guy in PA to buy SA Laser by Mito. Hopefully I get it! The Gaijin set is amaaaaaazing ... I'm really into japanese culture, so finding cool hiragana/katakana sets is great for me!
I'm starting to realize that this hobby is expeeeeeensive xD!
What are your top 3 keycap sets and do you have a preference about profiles?
Personally, I don't really have any huge interest in caps. The ones you mentioned look great to me, but I rarely buy any. If I do, I usually get some cheap Chinesium that seems to be of at least decent quality to me. Thicker caps are nice though, for sound purposes with certain switches. I like my caps to shine (wear smooth), and most of the market is matte PBT. if you like hiragana/katakana, have you looked at the red samurai set? I particularly like that one myself, but I haven't dropped the cash. That's basically where my knowledge ends on the matter. lol
I haven't put much thought into profiles either myself. I have probably tried some SA-esque profiles on vintage boards, and I think I have a Chinesium SA set somewhere that I never put on a board yet.
Also, any recommendation on Soldering Irons? I'm thinking on buying this one https://www.amazon.com/dp/B01MD12DYT/
That one has good reviews, and the portability must be nice. I prefer having more of a soldering station though myself, and the tips for that are expensive and don't follow any standard.
I usually use these. It takes standard Hakko tips, which are produced cheaply by many manufacturers. It has both a sponge and tip cleaner, both of which come in handy, and it has a place to the left to conveniently mount your spools of solder and desoldering braid. The coiled wire is a lot better way to stow a soldering iron while it heats up, or you work on something else, etc, as well.
If you think that you're going to do a lot of desoldering of existing boards,
this is also something to consider. It isn't cheap, but it saves you a lot of time. I can desolder a whole board with that thing in 10 or 15 minutes. There are more affordable options, but I wanted the best. I can't speak for how well the cheaper electronic pumps work but the manual pumps I have (which were cheap) were a nightmare. They clogged constantly, and getting just the right angle for proper suction was a pain. If you do get a manual pump, make sure it has a silicone nozzle, as that would help a lot with resolving problems with suction.
