Hi Eddie !
I have only started Eurorack a few months ago so take my advice with that in mind ;)
I have Plaits and BIA, both are excellent choices well worth the asking price in my opinion. BIA is particularly well suited for designing drum sounds and deep enough to allow for sonic explorations, for example I often use it to make bass lines. It definitely has its own character but can almost stand on its own. Plaits is even deeper, each algorithm almost feels like a new module. It has several sound engines dedicated to percussions as well so it's definitely not a bad idea either, especially since it will make your rig more versatile than a more percussion-focused module, Plaits also makes great synth sounds.
I have the Disting and while I find it super useful, especially to beginners, some people have expressed that they dislike the interface of this module. Indeed, cramming so much functionality in only 4HP, you end up with a non-negligeable manual to read, and some menu diving. I'm ok with it, others are not, something keep in mind maybe.
One thing you may want to consider is that the rack seems a bit light on the modulation/sequencing side. Sure, BIA and Plaits can stand on their own but to keep sounds interesting and evolving, modulation is key unless you have 15 hands. You would probably need triggers for your enveloppe generator (although they can loop on their own) and the sound modules, LFO's and modulating CV for other stuff as well, so it feels like the FH-2 could come short on that (although I don't know that module, I'm just counting the outputs). Did you count modulation sources and destinations as well as needs for triggers in your plan ? No one can decide for you but I would pay attention to this before making a final decision.
If you look for just another one to add, Intellijel Plonk is certainly one that should be considered for percussive rigs. I've only tested it a few times as I plan to buy one for myself but it felt like a very serious option for drum sounds. It's also a self-contained voice like the BIA and Plaits so it wouldn't need "complimentary" modules either. The WMD offering on drums also looks very tempting, although perhaps with more focused sound palettes. No experience with those yet, but I have Crater on order.
Speaking of "adding another one", the big weakness I see in your plan is the rack size. From personnal experience and many, many tales of similar situations I read on the web, buying a bigger rack than the one just big enough to host the modules you currently plan on buying is taking into account the possibility (rather likely to happen, I'd wager) that you will want to expand in the future, and you will need space for that. Something like the Intellijel's 7U in 84HP, it might look big to you at first but it's quite portable (one can definitely lift that one one hand), will give you space to expand and provide you with nice things like a 1U row to put some utilities (for example, Intellijel's Quadratt 1U is a great small mixer/attenuverter) and audio-MIDI I/O. It's just an example of course, the point I'm trying to make is that leaving only 6HP free on your first plan sound like you're going to hit that HP limit very soon, at which point you might have to sell your first rack already.
Hopefully all this will be useful to you ;)