Poco M6 Pro is perhaps the only option in the price range but much of the development is only happening in TG. Maybe some random Samsung or even Moto device.
As a side note besides Pixels and Nothing (QC devices at least) devices custom ROM development is quite sparse nowadays. Xiaomi devices still do see some activity but it is not as high quality as it used to be earlier.
Yeah right!
Installed my first custom ROM on HTC Hero in 2010, then HTC Desire 2011, then Samsung Galaxy Note 2012 (the first one), bricked Galaxy S2 trying to insert audio drivers which were apparently for Wolfson Audio while S2 had a Yamaha audio chip inside, then Galaxy S3, bricked a Nexus 7 (may still have it somewhere lying around) upgrading the OS manually. Bricked Xperia T , A Sony Ebook as well as a Nook Ebook Reader because of a messed up bootloader, lots of ROMs on Asus Max Pro M1. Currently running Custom ROM on a Galaxy N8010 with Boeffla kernel since 2012.
Even before this, modded my K550i to W6xxi, 'upgraded' audio drivers in my W850i!
Left custom ROM scene a long time ago because custom ROMs were fun in those days as they brought with them lots of new features and performance enhancements with them. (My HTC Hero CPU went all the way till 650MHz soemthing with a custom ROM).
Stock skins these days have almost all the features we need and hardware has become extremely competent unlike years ago when we were delighted to extract a few MHz more from the CPU!
In my time Snapdragon chipsets had almost exclusive developers' attention, Mediatek had closed bootloaders and shi**y performance hence developers didn't bother developing for Mediatek chipsets.
Maybe that scene has changed now but still I think Snapdragon are custom ROMs friendly.
You asked a question and I tried to answer.
Sorry I tried.