After considerable discussion, a couple winners seem to be coming to the fore. They're not as cheap as I originally had hoped, but that's life.
The following routers meet (or are close to) the criteria, and links to their recommendations:
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GL.iNet GL-MT3000 - Right around $100, available now, easy installation, plenty of horsepower - Thanks @qunvureze for the recommendation
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GL.iNet GL-MT6000 - This appears to be a great router, and is now officially supported in OpenWrt 23.05.3. Its price (~US$130, August 2024) doesn't quite meet the criteria, so you will need to decide if it's worth the extra money. Thanks to many people for the recommendation
Older Devices
- Netgear R7800 - Older model, well under $100 used, easy to flash, limited to WiFi5, only handles SQM to about 180mbps - Thanks @slh for the recommendation
- ZyXEL Armor Z2/ NBG6817 - nearly identical specs to the R7800. Thanks to @slh for the recommendation
Honorable mentions: These seem to be good routers, but don't meet all the criteria for newcomers:
- Netgear WAX206 - Looks easy to flash; similar power as Linksys E8450; may be available under US$100 on eBay
- Cudy WR3000 - Two-step install process is confusing for newcomers; reported to max out at 360-380Mbps with SQM; under $60 and otherwise looks interesting
- TP-Link AX23 - £53.99 on Amazon UK, likely can only support SQM at 100Mbps; nice link to the Device Page instructions - Thanks @hecatae for the recommendation
Linksys E8450/Belkin RT3200 - lots of power, highly-regarded, but has a confusing two-step install process for newcomers.No longer recommended for newcomers. This topic describes difficulties with flashing/recovering/bricking
If you know of other routers that meet (or almost meet) all five criteria in the original post, please add a new post describing the device. If you want to argue with the criteria, please start a new topic.
Thanks for all the good information!