[Redmi K20 Pro / Xiaomi Mi 9T Pro Questions & Answe] Toom_Raider:
I don't know if you made a decision yet, but in case you didn't yet (or for anyone else in the same boat) I want to give you my point of view. (Writing this on my T12 Pro)

Untill now there's not even a XDA device page for the T12 or T12 Pro or K50 Ultra (basically T12 with Snapdragon 8+ Gen1). So I don't expect the custom ROM support to be good.

Someone mentioned the long wait for unlocking the bootloader to be a reason but I doubt that's the main one. It seems like none of the recent Xiaomi phones have any really good custom ROMs available. I think MIUI not being the best base to build a custom ROM on could be a big reason for developers to stay away from it.

If you really prioritize good custom ROM support, why not go for a OnePlus or even a Google Pixel (or another brand with good custom ROM support)? I don't know how the ROM development is or will be for the newer phones, but the OnePlus 8T that I switched from had some good ROMs available. I switched mainly because I found a really good offer for it, €650 which I can deduct the tax from, and a free Redmi Pad along with it that I'll sell. If I manage to sell the Redmi Pad for €250 and the 8T for €300 I'll have made €13 on upgrading my phone.

Using the phone I'd say the only thing that's a big difference for me is the bigger screen, bit other phones can do that too (not many with a flat panel though, which I like about the T12 series). The Snapdragon 865 was already powerful enough for anything I threw at it including prolonged COD gaming sessions at highest settings, while staying cooler than the T12 Pro does. (Not that the T12 Pro overheats that much but it definitely does get warmer than the 8T).

The camera on the T12 Pro can produce better results, however for point-and-shoot I think the 8T will produce more consistent results because it's more optimized. Maybe Xiaomi will fix it with software updates but for now this is my experience with it, though I've only had the device for 3 days now so my experience with the newest MIUI is limited. I've already found one bug and multiple gripes with it though.

For me I'll probably use the device for half a year until I can renew my contract and find another deal for a phone that suits me better. For now I don't mind using MIUI; I immediately requested a bootloader unlock though to install the Xiaomi.eu ROM and root. I might also end up installing Pixelify on it if I get tired of MIUI (no experience with Pixelfy yet but seems an interesting alternative to custom ROMs), which would hopefully give me at least somewhat of a stock experience.

If you're happy with your current phone and the only thing you'd like on top is a good custom ROM, I'd say keep it for a while, and maybe try Pixelify to change things up a little.

If you really want another ROM but you're happy with the rest of the device, maybe try to get a similar phone used, preferably Snapdragon 865 or newer, with good ROMs already available. (I'd only buy used if you can pick it up locally and check the device)

If you want or can only buy a new phone, and prioritize custom ROM support, go for a brand that is known to have good custom ROM support.

If a Custom ROM isn't your main priority and you can get a deal like I did, go for it. In my case I can sell it for more than it costed me, maybe even a year down the line.