First, don't buy new phones. You're paying a massive premium to be first. Especially since you're going to flash a rom, which has a little risk anyway (I've bricked phones by flashing, though not for years).

I just upgraded from a 2017 flagship to a Pixel 5 (only because my cell company decided to stop it working on their network, when I can throw a different Sim in and it works fine). I was able to buy 3 Pixel 5's for less than you paid for your new phone. Which means I have a daily driver, a hot spare, and a test device for a little over $400.

If my daily breaks, I pickup my spare and swap the SIM, since I keep both phones synced with Syncthing. I don't even have to login to anything because that's all done. (I had 4 functional devices of my 2017 phone, they had become so cheap).

So pick a 1-2 year old model that you like the features, and pay far less for it.

Before (finally) coming to the pixel, I would look at the Lineage device list, then check those phones out at gsmarena.com and phonearena.com to see which I'd prefer, because Lineage has the broadest device support that I've seen.

Today I run DivestOS, a fork of Lineage with some changes to a few things. I forget now exactly what I preferred (I'd have to pull up my comparison spreadsheet), but average battery consumption is a staggering 0.5% per hour, with microg services installed and a couple apps using it. Consumption average increases to about 4% per hour when I'm doing a lot of intensive stuff - copying files over the network, using nav, watching a video, etc.