Thermostats for Use With Non-Smart Zone Controller

My sister-in-law and her husband recently purchased a split-level home with a 2-Zone HVAC system. The brain of the system is a Honeywell Truezone controller connected to two Honeywell basic programmable thermostats.

The lower level bedroom is normally occupied half-time (brother-in-law has joint custody of his son from a previous marriage) or on occasion when they are using the downstairs family room. The main living space is upstairs. They have several Amazon/Alexa devices (including one zigbee-capable Echo show) and some Hue bulbs (I think paired with the zigbee on the Show), but they're not exactly technically inclined.

They want to add some "smarts" to their HVAC system so that the basement is allowed to drift cooler or warmer when unoccupied, and held at setpoint when occupied. The perfect solution may or may not include a hub/controller. I say may not mainly because I would become their de facto support tech, a role I'm reluctant to take.

Is there a standalone smart thermostat [ecosystem] that can handle this cleanly? And not cost a boatload of money? For example, two Ecobees with motion/occupancy sensors could probably be made to work, but the cost would be quite high. And I'm not sure how it would handle "Sleep" mode because Ecobee typically ignores occupancy sensors (and assumes the space is occupied) when it thinks everyone is in bed. The new Amazon thermostat has an attractive price, but doesn't appear to support occupancy sensors (just Alexa hunches) and I'm not sure how it would work to have two in the same home. Anyway, I know it's a long shot, but I figured maybe someone here could think of an elegant solution that does not require a hub.

If the only practical answer includes a hub, I'm comfortable with what that system will look like in terms of hardware (two thermostats and some motion sensors at a minimum). However, I'll need to set up a system that "just works" and doesn't require them to use Rule Machine or edit device configurations within HE. Presumably I can accomplish this with a dashboard and the thermostat controller/scheduler, but that's one app I haven't played with yet. I'm also aware of the community HVAC Zoning app and could apply that as well. I'm just hesitant to put it together this way because I'm likely to get a phone call every time my sister-in-law feels a chill in the house.