Metropolix, Atlantix and Sealegs are very nice and powerfull modules.
I guess a great combo and I would love to have them. They are bit like the rolls royce of modules. High quality and features.
Its pricey, but awesome.
Yes Atlantix is a complete voice and this saves some modules. So this is a good idea. Because this gives a good ground and you may only add some small modules to get more out of it.
Utilities are very important and its always good to have the basic ground covered. But this can also grow with time.
It can be a interesting approach to start with these 3, to experience the limitations and to add was is missing later. I guess, modulating these 3 on itself could be fun.
Modular can be everything, it doesnt have to be complex, as long as you like it. And even with this 3 modules you can do very complex stuff.
In general, I would recomment to get a bigger rack at the beginning.
Its good to leave space to build around the Atlantix.
Because if you start with these 3 modules, you will see very quick, which functions are missing.
This will be probably small utilitie-modules like: a mult, attenuverters, a dedicated LFO, a mixer for cv, more random, logic modules, clock dividers
It always grows. So better leave enough room for it - or you have to buy another case again.
One 1U row is not enough space for that, in my opinion.
something like Pamelas Pro Workout could be useful.
Atlantix and Cascadia are different in character, so its a matter of taste, which you like.
Plonk is very cool. But be aware, its menu divey. I have Plonk and I love and hate it. I think 1 Plonk is enough, its very deep and complex and you can save and load drum kits on it, which can be sequenced. So you can get very much out of only one.