I'm in the market to replace my Samsung Galaxy S9+. While a root method was recently found for it and the S9, it leaves a lot to be desired (and breaks a lot of things). For my next phone, I'm looking for one that's super-friendly in regards to rooting and custom recovery (e.g. TWRP) and ROMs. I'm stuck with T-Mobile (US) for now, though, and the selection in-store seems slim, at least as far as phones meeting that criteria -- my best options there seem to be the OnePlus 7T, OnePlus 7T Pro Maclaren, or the Pixel 4XL, but none of those are exactly what I'm looking for: The OnePlus models, while I can get them in 256gb flavors despite the lack of expandable storage, but it looks like TWRP support remains unofficial for now? The Pixel 4XL seems like it'd be super friendly, but it has no expandable storage and the maximum available is 128gb, and I actively use the phone for a huge variety of personal, work, and media tasks, and 128gb won't cut it (and I don't want an OTG drive sticking out of the USB port, unless someone can recommend a super slim one > 256gb with power passthrough).
While I would like an obscene powerhouse of a device (say, an Asus ROG 2 with all the fixin's), practicality is the real motivator here. I don't mind if the phone is a gen or two back -- something from the same timeframe as the Galaxy S8/Note 8 up to now would be good. Or this upcoming generation, if you know something I don't. Definitely need >= 256gb storage, ideally with an sd card expansion. Dual SIMs would be nice, but not necessary. Ability to pay on installment (carrier, manufacturer, reseller, depressing mall kiosk, whatever) would also be nice, but is also not necessary. Most of all, I want to be able to install TWRP, systemless Magisk (with successful SafetyNet bypasses), and a variety of mods and ROMs (which itself requires an active community).
Anyone have any recommendations? Any questions or things I need to further clarify? Any online tools that can help me pick by selectecting and prioriizing requirements.