Thanks for trying it out! Enemy reinforcements are here to stay for now, but I intend to be more transparent with the player - give them a clear indication when the next wave will come so they can plan around it.
Right now I do not intend to introduce XP-based levelling during a run. I'm considering adding some character meta-progression - units who survive the run may unlock more stuff over time, but I haven't figured it out yet. Instead of XP I'd like to focus on progression through gathering gear - looted items will give units new abilities and player should be able to mix and match units and items to create diverse build - that is my intention at least ;)