Howdy and welcome! I think that lower row is a very capable collection. As they say, "You can never have too many VCAs", so I'd definitely recommend something like that Quad VCA. That's also a cascading mixer, so you can use channel one as a single VCA and use the other three as a mixer if you wanted to (or split as 2 and 2 for audio or CV, however you'd like!). Maths gives you a bit of attenuversion, but I'd still recommend having a standalone one too. The 3xMIA or TipTop MISO are both great for doing offset, attenuversion, and mixing. Matrix mixers are also super fun, and particularly for Serge style patching they make feedback very easy and fun to control. I think that Moog style mixer is also going to be good for adding character.
I personally don't really like the layout and labelling on Frap Tools modules so my preference is for Maths, but both are extremely powerful and helpful in learning concepts of voltage in modular.
So overall, you've obviously done your research and have a nice balance of modules selected in that bottom row, but would absolutely recommend getting an additional offset/attenuverter/mixer and the quad VCA. Knowing you already have the minibrute, I'd recommending getting the Maths, Wogglebug, Step 8, and Quad VCA next and play around with those. After that the Bifold is the least redundant with what you already have, and when you're ready for new sound sources move on to getting the rest of that audio path.
Cheers friend!