This is not legal advice and in no way relates to my employment:

Trademarks don't work like that. Prior use for trade is an absolute defence in most jurisdictions (though I'm not intimately familiar with USA) and is most likely a reason to invalidate the latecomer's trade mark. You're definitely not "sitting on [their] trademark" if you were using it for trade before them. If anything they're infringing your trade mark -- though that's unlikely to do anything unless there's 'passing off'.

Trademarks are usually limited by geography and by Nice class (famously Apple computers promised Apple Music not to do anything in the way of music business so as not to infringe Apple Music's earlier mark) so several businesses can use the same word as a trade/service mark as long as they're not likely to be confused.

The opposition period may well have expired for invalidation, I don't know how that works in USA.

Them badgering you, and a trademark attorney misrepresenting the situation could be reason for a countersuit by itself. Certainly I'd discuss that if they do enlist some representation.