Depending upon what you want to replace, there is the local option for Ecobee.
Set up Home Assistant on a RaspPi. Use HA's HomeKit emulation to add the Ecobee tstats. You now have completely local control of the Ecobee (with all the control exposted to HK). Add HA into your HE via HADB and you now have local control of the Ecobee tstats from HE.
There are several added bonuses is with this setup. The Ecobee room sensors have both a occupancy device and a motion sensor device. The later becomes a useful motion sensor that goes inactive in 60s. Their camera (which Ecobee sent me for free) works without their security subscription.
In HA, it looks like this:
I only use HA for it's HK emulation to locally connect my Ecobee tstats and my Aqara FP2s. I send the devices back to HE (via HADB) for all control/monitoring. But personally I only monitor my Ecobee tstats; I set up comfort settings and schedules internal to the tstat and let them do their thing.
I only ever used their Thermostat Groups when I would switch the tstats (I have 2) from heating to cooling or to off, at the transition points between seasons. I'll miss it, but it isn't a deal breaker for me.
