[General Topics] obs3rv:
Looking to buy a used, late 2018 technology Android One phone, probably off eBay UK around August/September/October. These are my priorities:

Android One (2018, so model would have come with Oreo out-of-box -- rather get an official update to Android Q down the line)

Unlockable bootloader (sadly this eliminates the HMD Global "Nokia" phones, which would otherwise be ideal. I'm aware there are third-party bootloader unlocks available for some models, but I'm also aware that some security updates have fixed the bootloader vulnerability that the third-party unlocks exploited.)

Availability of LineageOS, preferably official LOS that is maintained, today 15.1 or 16.0 (which is typically predicated upon having an official bootloader unlock)

3.5mm headphone jack (I have bluetooth headphones but prefer to use my wired cans without silly dongles -- unfortunately this eliminates the Xiaomi Mi A2)

regular (monthly or quarterly) vendor security updates preferably through 2020 (I wish Android One meant this could be taken for granted, but the Android security update scene is a burning trash fire.)

microSD card slot (yes, in 2019 -- I like to be able to do backups to microSD in case the phone completely dies and I don't trust network backup to work 100%)

Front and rear cameras, latter with flash and decent image quality. (I don't care about multiple cameras, and I don't need the best camera ever, but I'd like competent low-light photography and preferably optical image stabilization.)

>= 3,000 mAh battery (I'd love a removable battery but that choice essentially no longer exists)

At least FullHD (1080x1920) display, preferably 400+ dpi. (I can tell the difference between this and 720p at 5" and larger. My current and previous phones have/had QHD displays (1440x2560), but I don't think I can tell the difference between this and FullHD, while battery life and graphics performance tends to be worse.)

preferably at least a Qualcomm Snapdragon 636/660/710/Cortex A53 with clock speed of at least 1.6GHz. (I don't play games but would like a responsive UI, even with something like LineageOS which is in my experience often laggy compared to the stock OS.)

Screen size 5" up to (bezel-less) 6.2". I do have large hands but I'd like to be able to reach the top corners with my thumb with the phone held in a single palm.

Notification LED preferred. With bezel-less phones I understand this has become a rarity. However, without an AMOLED display, ambient display can get a bit annoying where the whole screen glows when a new notification is received.

What I don't care about:

Fingerprint sensor (preferable but not required)

Bezel-less display (I don't mind a chin and top bezel) or notches (if it has one, eh, if it doesn't have one, eh)

Multiple cameras (I don't like the fish-eye distortion of wide angle cameras and I'd rather have a single competent low-light capable main camera than a zillion cameras with different capabilities and a high price tag)

Wireless charging (tends to be slow, invites use of glass backs, though if it's available, fine)

NFC (honestly the only use of this I have is Google Pay, and I tend not to use it anyway as none of my credit cards offers it, only my bank's debit card which I don't usually use -- and both it and one of my credit cards are contactless, which is more convenient than Google Pay)

North American radio frequencies/CDMA. (I live in the UK and travel mostly to Europe, Middle East, South Asia.)

18:9 or 21:9 aspect ratio/ultratall. (I don't usually watch videos on my phone.) Fine if they're this ratio, fine if they're not.

Water resistance. I tend not to do things like drop my phone into a toilet. It would be nice to be able to use the phone in the rain, but not at the cost of things like a headphone jack, or annoying port flaps like Sony used to put on their water resistant phones.

I have a crippled LG G5 (bright squares along the left and right edges of display, damaged motherboard so GPS antenna isn't connected, non-working proximity sensor, non-working LED flash/torch) and a slow Wileyfox Swift (can get unpleasant to use with somewhat laggy LineageOS 16 on an old Snapdragon 410 and a defectively-designed microUSB port, and a bad camera).

My thinking is -- the most important thing for me is availability of security updates for the 2+ years I intend to own the phone. Being a used phone, that official vendor update lifetime is likely to be reduced. So I'd like to have vendor updates for the first ~year and then after that, a ROM like LineageOS that has nightly/weekly/monthly builds and merges in upstream changes including security updates from Google.

Next, I'm a bit old fashioned, so I like my headphone jack and microSD card (and while this is no longer possible, a removable battery).

I expect to use the phone as my primary camera on the occasion I take photos, and I tend to take photos indoors, so preferably good low-light performance. (Not the best camera, but a good one that works decently with low noise when there isn't much light.)

And I'd rather have a good display (not the best display, but not so low-res I can see individual pixels). I don't need a phablet or a huge device, even down to 4.x inches is fine, up to modern bezel-less 6.2", so I can operate it one-handed.

I'm looking to pay about £150-200 for a ~1 year old used phone towards September this year, so phones that go used for about £180-250 now or new for about £250-400 now would be suitable.

Any suggestions? Or is this a pipe dream?

The Nokia 7.1 seemed to come the closest, but for lack of official bootloader unlock, and the lack of a notification LED (which I suspect is as dead as removable batteries are). As a result, the only way to get a custom ROM on it is via a third-party bootloader unlock, and there's essentially no ROM development happening specifically for this phone. (There may be Project Treble GSIs, but how much testing would they have gone through for this model? How many of them are officially supported by ROM developers and get regular security updates?)