Yeah, Avatax prices are crazy for a small operation. I'm in a similar boat and ended up using a combo of manual setup and a cheaper API-based service. I use TaxCloud, it's not super flashy but it's way more affordable and handles multi-state rates decently. No native Abantecart integration either, but their API isn't too bad if you're comfortable setting it up or can get someone to help.
And yeah, your accountant's right, once you hit nexus in other states (economic or physical), you’re supposed to collect there too. It’s a pain, but ignoring it can come back to bite later. Keep it lean early on, but set yourself up to scale without headaches.