All of the recent Behringer stuff I’ve gotten to try is pretty good, definitely worth considering but most people online are too cool to admit it.
If a mono synth works for you, I’d take a good look at the Behringer Neutron. A friend of mine has one and it sons great and has a LOT of patching options.
If you can afford a bit more and want somthing that’s modular but self contained, The Kilpatrick Phenol is really underappreciated. It was discontinued recently but they’re still easy to find. Not many good demos of it online, but it’s a really nice sounding synth, the stereo line in would work really well with the Octatrack, it has really stable oscillators, nice (and unique sounding) filters, and I actually really like the digital delay (it’s very lo-fi by design and some people don’t like that but it sounds great to me, completely unlike anything the Octatrack’s delay can do so they complement each other well). Plus BANANA JACKS and a halfway decent layout, which already puts it above 90% of Eurorack modules I’ve tried or owned. It has enough modules in it that you can easily use half of it as a simple, MIDI controlled, one oscillator monosynth and still have all the building blocks for an entire second monosynth (only one channel of MIDI to CV, though, so youunfortunately can’t patch p two independent mono synths with MIDI control, and also there’s only one LFO, but it’s still very usable for melody+abstract space noises). I’ve been using it in a mixer feedback loop lately and that works well.
If you’re mainly looking for a synth to play sequenced melodies I wouldnt’ recommend it, but if you want something that can get into weirder modular territory (an sample material for the OT), be a solid monosynth, or do a bit of both at the same time, all I can say is I chose it over the 0-coast 4 years ago and never had any regrets about the choice. Plus the fact that it’s underappreciated and has distinctive sounding filters helps make it stand out from the crowd.
It’s not the best value-for-money synth out there (that would be the Behringer stuff easily, if we’re sticking to analog - there are a lot of nice digital synths in the sub $300 zone) but nothing I’m aware of sounds like (or has envelopes like) a Phenol and if the Dreadbox stuff is in your price range you should definitely consider a Phenol.