If you're looking for wildlife and birds, I really wouldn't worry about a prime being a nuisance, if you feel you want more reach than the 55-300, then you'll probably want more reach even with a 400! I find I very very rarely want less reach than my 400 offers when out shooting, in fact I almost always want more and end up cropping a bunch! (Don't forget you'll still have the the 55-300 to cover stuff in that range.)
As far as a recommended 400mm, if you don't mind manual focus, I'd certainly recommend the Pentax A 400/5.6 - that's what I use for my super tele needs. On the other hand, manual focus can be a bit limiting, but think of it as a challenge. (An often recommended combo that's modern and AF is the DA*300/4 and 1.4x teleconverter, giving you a 420/5.6 - I can't speak to this, not owning it, but it is something I'm looking at acquiring in the future.)
Keep scouring Ebay and PF marketplace, among whatever other used sites you look at, and if you see an A 400/5.6 at a good price...