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)

You can buy the sensors of aliexpress and link them with esp8266 via esphome (a home assistant addon) if you’re into it.

It’s fairly easy if you’re a tech savvy and will cost you next to nothing like 3 dollars each node which can have multiple sensors (temp / humidity / light / relay / etc).

It’s better to make dumb devices smart yourself to avoid being locked out. I literally cannot change the water level in my vacuum mopper if the server in china is down.