This is a suggestion but not a recommendation because I have not used this device:
There’s a similar discussion (about the termination of Nest’s API) on the openHAB community forum. Someone there suggested this thermostat as an inexpensive substitute with support for heat/AC/multi-stage. About US$85.
I’ve been using a HAI Omnistat/2 RC-2000 for almost 10 years. Bought it used from eBay. Wrote my own driver for it and it communicates via RS-232 (although a zwave version is available). It has an extensive API but the silly thing doesn’t report any manual changes you make (like adjusting the temperature) so the driver has to continuously poll it for status. Anyway, I don’t imagine too many hobbyists choose this one anymore but there it is.
Proliphix made two ‘network’ thermostats (Wi-Fi) but has discontinued them (Home Assistant supports them). They still offer their higher-end IMT550c/w which is even available in a wired Ethernet version. Apparently its API is similar to the one used by the discontinued models. Like the HAI Omnistat this thing still commands a high price even for used ones.
EDIT
I just noticed that @silvrr recommended the same zwave thermostat here:
Anyone interested in it now knows who to ask.