Hynman’s sysupgrade package can be flashed straight from the stock firmware. He has a nice set of features, it’s a clean GUI - makes it easy. I’m personally running his latest master build.... I like having the latest features and have found it stable. “Stable” builds like 19.07 are a branch off of master from July 2019. They get security updates but no major new features.
As long as you pick the correct country code - It’ll guide you to the safe transmit power settings for your adapter. The default setting is ok- I’d pick the transmission setting that best fits your needs.
Speed is both throughput (mbps) and latency (how many ms do I have to wait). If your speed throughput is in the low 100’s - that’s good. Check your latency and see if OpenWRT can help. Latency reduction is a major benefit of OpenWRT - you might benefit from SQM to lower the latency of your internet connection “bufferbloat”. The r7800 has plenty of CPU for your speeds. Use DSL reports with the high resolution bufferbloat setting and see if bufferbloat is a problem for you.
Here is a recent thread on SQM and the r7800: