In your situation as your speakers are easy to drive and you’re enjoying the sound I would not recommend adding an amp. As mentioned, you’d still be stuck using the preamp section of an AVR, which will continue to be a big bottleneck to significantly better 2-channel performance. For better stereo in the longer term you’ll want to focus on adding a good stereo integrated amp or separate amp/pre (via the Anthem’s preamp outputs) that’ll allow you to completely remove the AVR from the 2-channel signal path, which is ultimately what you want. And then upgrading your speakers, source, and cables would make further significant improvements. The good news is you can do this in incremental steps and get huge improvements along the way, so keep reading/learning, go out and hear as much equipment as possible (going to AXPONA is highly recommended especially if you’re new to this and have limited access to good dealers), and enjoy the process!
As for HT, you’ve got a good AVR that should serve you well for a long time, and adding a center speaker can help especially if people will be sitting off center. If not and it’s just you in the sweet spot, with your equipment and proper speaker placement you should be able to get a good phantom center image so if you’re happy with that as is you might be able to skip a center speaker and put those funds toward better 2-channel performance. Hope this helps, and best of luck.