For guitar pedals - go with Strymon's AA.1. Mainly because you can use it as two mono loops with a splitter (1/4" TRS to two 1/4" TS).
I'd check out Intellijel Quadrax or Frap Tools Falistri as an alternative to MATHS.
I had rings - not my thing ... but for what you are doing, it's perfect. I can only imagine what you could do pushing a cello through it!!!!
Can't go wrong with Plaits and Clouds. I'll never part with mine.
If you wanted to record grains of your cello or field recordings - look at Instruo Arbhar. Clouds might not be a greatest choice for this. Or even the Lubadh - if you wanted to get more into tape loop type stuff.
For sequencers - it depends on what you want - for more generative stuff, look at Marbles as a starting point. If you want to program specific movements - Rene2, or Intellijel Tete/Tetrapad - which can be programmed - but also do generative/cartesian stuff.
Lots of good videos from Make Noise, Intellijel and DivKid on all three of those modules.
I had Marbles - but I'm not a big fan of that type of randomness. Tete and Tetrapad are more my thing.
If you wanted to start small - you need something to make notes - get Marbles, plug it into an envelope generator (maths/Quadrax/Falistri), with pitch into Plaits (with internal LPG/VCA) and rings (plucked), mix it with Veils or Intellijel QuadVCA and then pass it into Clouds. Instant ambience.