On top of what @jamesh already listed, there’s the backend (store) which has a bunch of features of its own (phased rollouts, repair assertions, gadget/kernel, some telemetry?, CDN and so on) that aren’t obvious if you only look at snaps and snapd. Some of it may be available only as part of the brand store offering, I’m not sure of the details.
At the end of the day it’s a matter of whether you are ready to roll out and support something custom, or use something like Hawkbit/Mender/whatnot instance with your favorite updaters, pay for some SaaS offerings (hosted Mender, Balena maybe?), use an end to end commercial solution if your budget allows it or go with snapd & snap store.
Disclaimer, I used to work OTA solutions, eg. CarSync which became Wind River’s Helix, and Mender before joining Canonical. It’s fun but the costs of rolling out your own with scaling and support are not always obvious.