Welcome back!
This should be a stable setting, since the CPU's are always running on 1700MHz, as long as they don't scale up/down, you're most likely not seeing an unexpected reboot because of CPU scaling.
No you shouldn't tick those boxes in the firewall section. The hardware offloading in that section is for Mediatek devices I believe. NSS builds on this platform (IPQ806x) have hardware offloading always enabled. The software offloading is kind of pointless if you've already got hardware offloading. The software offloading is a compromise if you don't have hardware offloading in your device. Hardware offloading is always more performant than software offloading.
I have AdGuardHome running too on my R7800. I've got a USB-stick inserted. All logs and config is stored on that. There's a great tutorial for that too.
I've learned how to do that when I watched an excellent video on that topic on YouTube from @onemarcfifty. He has more great video's about OpenWRT by the way, you might want to check them out.
Sounds like that is perfectly doable when you follow the YouTube video I mentioned above.
About your question on how to achieve such a setup, you might get more help involved when you open a topic about that in case you can't get it working in the Installing and Using OpenWRT section.