Howdy!
This is a pretty open ended request, so we might need a bit more detail to be able to give you adequate advice. I know it's not specifically Eurorack, but I would really recommend the Behringer 2500 clone as a very reasonably priced all-in-one extremely versatile option. You can also mount that in a standard rack and add a row or two of 84hp rails to expand with things you think you're missing. For drone/ambient I'd recommend some more sources of modulation and maybe some interesting delays/reverbs (if you're not already running it through pedals). Maths is a great addition to the 2600 for envelopes and more complex modulation (also highly recommend the illustrated supplement for Maths, you'll learn A TON about voltage and how to push it around). Updated clones of Clouds (like the Monsoon) are pretty "classic" eurorack nowadays too. Sort of an instant ambient playhouse.
This might just be my personal opinion, but I don't think synthwave type sounds are really where modular shines. When I think of synthwave I think of more typical polysynth options, but the beauty of modular is you can always run a keyboard synth alongside it! (or even INTO it!)
Any more detail on things that are drawing you into modular or sounds that have been inspiring you would be greatly helpful to give you more specific direction.