I'm surprised to hear you say that. I've played the most demanding Android games on the Pixel 3a with no issues. I've never experienced anything but a butter smooth UI on Graphine or Lineage to be honest. The battery life has been all day for me even when using GBA emulators for multiple hours a day.
I agree the default camera app of Graphine isn't great, but it's picture quality better than the iPhone I came from (iPhone SE gen1)
Can you install GCam as apk from somewhere? Will it work? I use GCam on the default Android (8) on my Nexus 6P and it works well. I am thinking of upgrading to Pixel 2XL or 3A and install Lineage OS with GCam, so I believe it would be a much better experience than the default ROM on a Pixel. But I have no idea whether GCam would work in LOS.
You should be able to get GCam via Aurora Store by setting the spoofing to a Pixel device, but newer versions of GCam check for something that cannot be spoofed with an app (issue #22 in above github) so you have to get a modded GCam app if you want to user newer versions and use an android rom that does not spoof this.
I agree the default camera app of Graphine isn't great, but it's picture quality better than the iPhone I came from (iPhone SE gen1)