Try with LineageOS4MicroG docker image (you can build a stock LOS with that). Anyway after the first time next builds should be faster. The biggest problem will be compatibility, probably the new LOS you build will be incompatible with the older kernel version built for your device, and you will have to (at least) fix some bugs and find some workaround

I highly doubt it. The custom ROM/OS projects have entire teams working on them, and they still can end up with bugs amd problems.

You're overlooking the chance that you missed something critical, or aren't aware of some weird driver interactions with your phone hardware that could end up bricking it (unlikely, but possible). At the very least I'd be kind of surprised if your "compile it yourself" works on the first try and doesn't need some adjustments to actually run on your specific phone hardware.

Compiling your own fork of a program is one thing. Doing that with the OS is an entirely different ball game with an order of magnitude more potential complications to consider.

You can snag a Pixel 6a on Swappa for about $100. I looked into compiling the OS once, and it wasn't worth it for me (on an i9 w/ 16GB RAM and a SSD, still would take days).