Very hard on that price range. Above your price range sits the Asus Zenfone 10 (the 9 is really good as well), which covers a lot of your points. The 10 allows you to disable everything and have stock Android experience. Not bad ! Not sure the 9 had that.

I like the pixel series and recently went with the 7 pro. Wanted to wait until the 8, but my 5 had other ideas.

I think your best bet might still be a pixel (probably a-line), since that would be the path of least resistance for software support and custom rom development/rootability. Another fun option is ASUS with great price/performance and a huge battery (I think it's fairly repairable too?), but it doesn't seem there's a community for custom roms. Here's the main possibilities and downsides as I see them:

Pixel: a little pricey, not great price/performance, not sure on repairability, weird issues with extra sw features apparently getting worse over time

Samsung: forget about custom roms, again not great price/performance at your price range, repairability okay but not good

Fairphone: bad performance, I don't believe there's any water resistance and build quality would be a concern for me, unsure about software support but I'd expect good custom rom community

Sony: not a large user base, so I'd assume there won't be custom roms. I think Sony hasn't committed to the same software support as other big names. I'd be worried about availability of replacement parts for repairs

ASUS zenfone: out of your price range. Bad track record for sw support

Xperia 10 V doesn't have 256 GB. If you're ok with 128 GB you can have a look at the Xperia 10 III too, the differences aren't that great (and it's cheaper).

The III has already reached the end of upgrades whereas V will receive another 2 years. On the flip side you can install LineageOS on the III (and probably the V too in the future, check out GSI support).

The thing to keep in mind for LineageOS is that the GSI ROMs will jump to a newer Android version, but the hardware image (drivers etc.) remain stuck on whatever the manufacturer last issued. So they are an upgrade alternative, but there may be incompatibilities in the future, so they're not a perfect one.

The V has eSim support if that's important to you.

Another OnePlus. Buy a brand new but not the last model. @400€

Hey OP ! I'm digging this back up, I found a smartphone that might interest you (with a few caveats) : The Nokia G42.

It's an interesting one ! Built to be durable and reparable. It's a smartphone, but Nokia worked with iFixIt to design this model; you can remove the back cover to "easily" change the battery, the USB-C port and the screen. It has a headphone jack (even if you don't care), a decent screen (90Hz, 720p though), has expendable storage to 1To, approximately 2 days of battery life (Nokia claims 3 days), it's "cheap" (can be found where I leave to around 230 euros) and has take decent photos (for its price). Not sure about rooting this one though !

The main issues from my point of view are updates (3 years of security update is OK, but 2 years of guaranteed Android updates is short), and the processor, which is quite weak. Not sure how it will perform in newer version of Android.