I think highest/easiest ROI would be devices that alert or avoid bad events.

• freezer/fridge temp monitors to avoid food going bad (circuit failure, door cracked open, part failure)
• water leak detectors (water heater, bathroom, sink)
• open door alarms ( garage door left open overnight, fence door left open that lets dog out)
• doorbell camera/microphone (unwanted visitors)

• I am sure you have at least a couple of instances in a week where you go “damn, I forgot to switch the lights”.
• You are tired on your sofa and wanna watch a movie but the lights are too bright.
• You want to simulate someone is home so you don’t get robbed.
• meeting mode: warning them with a red bulb in the living room, and lowering the TVs volume all in just a click, while ensuring that your robot vacuum stops cleaning and resumes later.
• get a warning that your room’s humidity level is high, and directly offering a button for launching AC.

All those things I have not done, yet. I don’t even have the Zigbee dongle. But surely through the following months and years I will find little things that I could do that sum up to a lot of comfort and saving time.

Also, relay switches and sensors are so cheap it can simply be a fun hobby.

The only smart appliances I have ATM are smart light bulbs. These also partially contribute to saving energy (eg turn off lights when nobody is home / in the room), but even if that is not your priority, you can set up nice automations to soften the light as it becomes darker. I also use HA with Rhasspy to voice-control the lights.

As for fridge contents tracking, I wish there was a usable solution to what you describe. I have tried Grocy (can be integrated with HA), which looked nice, but I eventually stopped as it took a really long time to maintain. After each shopping, I had to input everything I bought (almost an hour usually). Then, each time you use something (eat a bun with some cheese, finish a bottle of milk, open a new sachet of some seasoning, etc), you must ‘consume’ it in Grocy. 99% of the time I forgot or was too lazy (once again, it just takes too much effort) and therefore the stock database was always out of date, making the whole thing useless.

Other things: I haven’t tried yet, but I also want to acquire some cheap sensors for temperature, and perhaps other things that I can’t think of right now. Maybe turn on the AC for a bit when it gets too warm (and somebody is in the room)?

Personally, that isn’t how I think about a smart home system. There isn’t a need to do major changes until maybe you need to get it replaced anyways. Starting with things like lights, a few shades, door sensors, are good ways to start. The biggest question is what do you want to get out of it?

Doing temperature monitoring in each room of the house. It really lets you work out where you’re losing heat, or failing to cool in the summer, and keep things more comfortable.