At that size of case you probably want to be very precise as to what you want to do with it, all the modules you have picked out are cool, but they don't really compliment each other on their own.
The west pest probably takes up the space of what you'd want to reserve for all of your sound sources in the future, I put way too many into my first rack, and I'm still taking them out to make space for more essentials.
Maths is well regarded, it can do modulation, attenuation, slew and lots of cool stuff... but it cant do all of that at the same time, you will likely want a lot more than Maths and the lfo in west pest to move things around.
In a case this size I'd use stackables or passive flying mults instead of putting a mult in the case, but also... there are 2hp alternatives if you really want one.. or even better something like Links would do a lot of work in the same space.
It might be worth looking at what specifically would work well with the west pest, e.g. filter, more modulation, vca's and utilities to give you more nuanced control over it, a sequencer if you aren't enjoying the built in one, maybe one effect modules (the FX aid is a great little one to look at)
Looking further ahead I'd look at what you'd need to replace some of the functions in the west pest when you come to swap it out, e.g. clock (PPW?), sequencer (Misha?), Midi in (black Midi - CV?), LFO/Modulation/envelopes (Maths, but you likely want more of these anyway, look at the ochd, quadrax, tilt, triple sloths etc for inspiration as well as any function generators, envelope generators, and modulation sources you can find, PPW will do some of this, but its not quite as straightforward).
I'd also consider if you want any sequenced modulation (a second sequencer) or generative sequencing/modulation (e.g. marbles clones)