While I agree that the monochrome kit is very lacking, I am leaning towards following the keys offered in blocknet from gmk laser. I have these keys and while they couldnt cover an entire 75% keyboard, i am confident it can cover atleast 65%. I will be making a mockup for you guys to check out when i am able to. Just need to review some layouts to ensure compatibility to 65%.
I would say that is the best way to go about it as mentioned in my previous reply - it covers basically anything up to and including 65% as you said which is a big portion of the market, excluding ergo and weird 40% stuff obviously, whilst keeping the price fairly accessible.
Of course you could also include the necessary keys for 75%/TKL as you had in your initial mockup last page, that'd be important for many people I'm sure, but it really depends on the quote you get - coloured modifier kits like the ones for SkIIdata and Terminal R2 were about the same size as that would end up being and cost 120 and 100 dollars respectively. Laser's Blocknet cost $45, albeit at a ridiculously high number of 900~ sets sold, but it was still only $55 at 100 which is very good.
As long as you're confident in the base kit's ability to pull good numbers, trying to make the weeb alphas + monochrome kit as appealing as you can as a possible, actual alternative rather than just add-ons is really important.