Since I brought it up, I think there are two aspects to this. One, when you buy an expensive thermostat, it comes with a snazzy touch screen and a decent user interface that allows both quick basic use as well as access to all the (many) advanced features; e.g., look at the manual for one of those Honeywell jobs (disclaimer: chosen as an example; I neither endorse it nor claim it is the best on the market for any purpose). Working from scratch, either you download a ready-made app that does what you want, program your own, or make do without one (eg if you only need to change the settings once a year maybe you would not mind if there were no touchscreen).
Second, building a thermostat from scratch, like out of a raw temperature sensor, humidity sensor, wires, and relays, while certainly doable, might be said to require some basic electronics knowledge, maybe also engineering knowledge of PID control and similar, plus you would want to package/mount the thing so that it looks nice, without loose wires visible everywhere. I actually have a pre-built “smart thermostat” (something like this, not that exact model though, mine is a lot less smart, what it does do is accept a temperature set point (from your program) and turn a heating element on and off to maintain that temperature. I am using it on a little oven, though, not to heat+cool a house; don’t know how complex it would be to tap into your new house’s HVAC system (it may even be easy if the house already has sensors and heating/cooling relays and all you have to do is replace the “brains”).
If all you need to do is swap between two schedules (with the push of a button at worst) it may be more efficient to buy a pre-built unit unless you love and feel comfortable with DIY stuff or can’t find an existing one with that feature.