There is a Home Assistant app for Garmin watches that is pretty usable. The flashlight is surpassingly, hilariously, one of my favorite features though.
I would very much like this too! Especially if it can count distance/steps from other kinds of activities like biking, stationary bike, maybe even swimming.
Neat. I did something similar but simpler. I put a cheap zigbee motion detector in the mailbox and hooked it to a routine to toggle a flag and trigger a notification. Yours sounds like more fun though.
It doesn't seem able to display in my silly American units. At least not Wind speed or visibility. It would also be amazing to have current temperature, wind, and humidity on a single widget.
For image hosting I would look at Immich. It aims to be a full Google Photos replacement. It isn't quite there yet, but it is quite featureful and rapidly improving.
Look at it as a hobby. You will sink as much money into as you want. I started with a cheap mister coffee espresso machine but now have a pretty nice E61 group head machine and jazzed up grinder and all the various coffee toys. It all brings me joy. The morning coffee ritual is what gets me out of bed.
From a cost perspective, if you are the kind of person who drinks 3 lattes from a cafe everyday, then even with the cost of a relatively nice machine and grinder will be paid for with the difference pretty quickly. If memory serves, you save about $4 per cup which adds up pretty quickly.
What phone functions are you interested in? In theory if you can control it from HA you can control it from Assist. Sometimes it requires a bit more elbow grease though. I don't use timers much, so I haven't tried that. I don't see why you couldn't create a custom sentence and intent for it though.
I've already replaced It with Home Assistant. Wake words are still shaky, but everything else is at least as good if you're willing to configure it. At least for my use cases.
I have mine working with Navidrone without any issues.