Thanks to the info in this thread, I got a working AirGradient Pro with a Lolin C3 Mini 2.1.0!
It still took a bit of fiddling, so leaving this here in the hopes it may help someone else.
Issue 1: Couldn’t flash with esphome web.
After flashing with the web flasher, the C3 refused to boot and get stuck “waiting for download” (regardless of what firmware I tried to flash, and no matter how many times I tried).
solution
Use web flasher to “prepare for first use”. Flashing works, but the subsequent boot does not while plugged into the computer’s USB (some error about improv serial, and stuck at “waiting for download”).
At this point, disconnect from computer’s USB, and connect to a power only USB. The C3 will boot that way ¯_(ツ)_/¯ . Now connect to it using its wifi AP and use OTA to flash airgradient firmware.
Once flashed OTA, it will boot happily whether it’s plugged into a power-only or a power-and-data USB port…
Issue 2: Unreliable wifi
I was pretty dismayed with the C3 at first - just couldn’t get a stable wifi connection. It would connect to a far-away AP instead of the one right next to it, randomly disconnect, and constantly reboot after losing the connection. This is, apparently, a known issue (github dot com/luc-github/ESP3D/issues/1009, apparently I can’t post links) with the C3…
solution
Reduce tx power. (This is already mentioned by several posts here, but a bit buried so I missed it)
wifi:
power_save_mode: none # Not sure this is needed?
output_power: 8.5db # Fixes slow connection and constant disconnects!
Thanks everyone who’s contributed to this thread and helped me get this working!