Semi-modular, and in that case eurorack-compatible, should be of interest to you then because you can expand once you master your synth.
There are tons of decent synths available in the 300-600€ range and you'll get a lot of recommendations from everyone. I'd have recommended a Moog Mother 32 for instance, others an MS20, others a Behringer Neutron, others a Microfreak or a Minibrute, etc... They're all good recommendations but I think ultimately, it all depends on what you'll find to be the best work flow for you, or the easiest to grasp (in addition to sound, of course.) Some people start with the No-Coast and love it, I personally find Make Noise gear to be very very nonsensical layoutwise. But maybe that's what will work for you.
Watch as many videos as possible before you commit to any of the recommendations you get. Look for sounds and workflow. If it seems to make sense for you, go for it.