I'm new to OpenWRT, too, and I'm also installing on a RPi-3B+. I have a working and reasonably well optimized system. I read/watched MANY articles and much of the documentation about installations and found the same confusing hodge-podge of GUI + command-line instructions you did. I managed to cobble together something that worked, anyway. At least I think it does.
Yesterday I found the tutorial by "aporetic", here HOWTO: OpenWrt on the Raspberry Pi 4 , to be most helpful, and it may give you the guidance you're looking for. "aporetic" recommends against using the Pi internal WiFi system, but I've found it satisfactory for my purposes (not gaming, just serving several devices; will try video streaming later today).
In particular, my stable, "successful" setup had very sporadic ping response times from the Pi router -- significant std dev in times across 100 pings. "aporetic"'s performance tweaks resolved that. But I also converted to using the "Legacy" setting for the Pi's WiFi system (2.4GHz rather than 5Ghz), and that may have helped.
My setup is that we're moving to a different state, and I needed a temporary router that I can take to our new home when we travel there to get set up. So I plug the Pi into an Ethernet port as the WAN connection, either on my ASUS router in our old home or into a cable modem in our new home, using DHCP to get an address on the WAN side, and I use the WiFi as the LAN side. I've specified the IP on the LAN side to be 10.9.8.x/24, and it offers DHCP service to connecting devices.
I still have some problems with initial connections after a reboot, but after much reading, I think that may be due to impatience on my part. The Pi doesn't have an internal clock, and apparently OpenWRT on the Pi has to be given enough time to sync time on the WAN side before it really functions.
Again, my experience is very limited, and I have a very limited use case, but I'd be happy to brainstorm with you as you work your way through this if it would help just to have someone who's going through the same stuff.
Ah, I tested video streaming over OpenWRT running on Pi-3B+ using the Ethernet connection to an ASUS router for WAN and using the Pi's 2.4GHz WiFi for LAN, and the video streamed quite nicely. So I think this setup will work just fine for my purposes.
David