Not sure it quite compares to the phone custom ROM community in size but there is an active community. I've reflashed cheap wifi IoT devices which otherwise wanted to phone home with Tasmota[0] which seems like a relatively active project. There are other similar efforts too. You could also consider eg home assistant, openhab or domoticz to form part of this type of community and they're very active.
And I really have to think that the more companies add these misfeatures, the more effort is going to put into alternate ROMs/firmware for more consumer electronics.
0. https://github.com/arendst/Tasmota