(Apparently the reason is LineageOS' lack of / in-development state of A/B partition support which would enable safer upgrades.)
I'm somewhat surprised they aren't including the OP6 in the initial supported devices, but there could be lingering bugs with hardware. (Blurry photos, for instance.)
(I'm aware that I could flash it manually but I'm far too lazy for that)
A used Google Pixel XL in good condition is currently priced around $100-$130 on eBay, and supports all U.S. carriers. This model is on LineageOS 15.1, but should be upgraded to 16.0 once a technical issue is resolved. The Google Pixel XL's PassMark rating is just over 40% higher than that of the Samsung Galaxy S6, and it packs a good camera and an above-average 3,450 mAh battery.
https://www.gsmarena.com/google_pixel_xl-8345.php
https://www.reddit.com/r/LineageOS/comments/at3bf5/_/egzcoxb...
https://www.androidbenchmark.net/device_list.php
https://www.dxomark.com/google-pixel-camera-review-retested-...
If you're looking for something more recent, a used Moto Z2 Force is in the $100-$150 price range, but comes with compromises. Its PassMark score is about twice that of the Galaxy S6, but its camera is weaker than the Pixel XL's and it only has a 2,730 mAh battery. On the other hand, the Moto Z2 is a modular phone and is compatible with a range of Moto Mod accessories, including extended batteries. The Z2 Force already supports LineageOS 16, and the unlocked version works on all U.S. carriers. Its screen is shatterproof, but susceptible to scratches, so a screen protector is highly recommended.
https://www.gsmarena.com/motorola_moto_z2_force-8693.php
https://www.dxomark.com/motorola-moto-z2-force-review-motoro...
https://www.motorola.com/us/moto-mods
OnePlus phones are also good options, but they're mostly limited to GSM carriers. If you're in India, South Korea, or another country with compatible cellular bands, the Xiaomi Pocophone F1 offers excellent value with a fast processor, a good camera, and a large battery.
And this might not be an issue for everyone but fact is that in my own circle of friends I know of three Nexus 5X phones that have died to hw failure. And one that is acting strange.
But I personally still have my old 5x with LineageOS and no issues at all. I love it.
What I would go for is one of the phones that came with stock android 2-3 years ago. Buy it used it perhaps from a re-seller that still has one in stock.
Pixel for example has a really nice feel in my hand and is pretty cheap.
I also don't know why a faulty / for parts S6 still goes for ~$150 here in Australia.
Remember that even with LineageOS you're still at the mercy of your OEM to actually release security updates for the vendor layer and Google is still by far the most active when it goes to security fixes.
https://www.androidauthority.com/xiaomi-redmi-note-7-pro-960...
Xiaomi phones have pretty good custom ROM support.
I looked on /r/LineageOS and found some older posts (a year plus) that claim to give some insight on this. Those might still be fairly accurate, but the same conditions that would cause me to need a guide also mean I wouldn't know if one is still accurate after a year. :/
I would consider putting it on my current phone since it's close to end of life with the shipped Android (Samsung S6, getting slow), but it only seems to support 14.1, which just went unsupported earlier this month.