My advice if you want the full experience but don't want to expand too much? Get a semi-modular synth and then just buy modules to augment it. A Pico System III is a great one because it has so many pieces that can be used in different ways. The Quadrantid Swarm is a good one if you want something less traditional but still very musical. There are plenty of good options, and they can save you space in your rack.
Also consider something like an Empress Zoia or a Poly Hector (or even the non-Eurorack versions, which have MIDI outs and can interfave with the controls on your Sub37 in deep ways). One of those and some attenuation/mixing for hands-on control might scratch the itch for a long time. The Hector is also full of Mutable Instruments modules because they're open source, so you get a lot of classic modular in there.
There's also stuff like Ornament and Crime or the Disting modules, with lots of different tools in them that you can try. Not super hands on, but you don't always need that (or you can use attenuators to change this). If a function in there makes you want to go deeper, you can get a module from there.
Have you tried VCV Rack? I find it good for planning systems in addition to making music on it. You can get a sense of how things will sound and decide what you want more or less control over.