There is a good discussion of --read-data-subset here:
In my opinion, all of the major cloud providers are reliable, and a lot of the smaller ones actually use the major ones. I think reading your data 4 times a month is excessive; even once a month is excessive. Personally I don’t bother, but I back up to two local NAS boxes plus the cloud. I occasionally download something from each and have never had any issues with restic recoveries.
If you really want to do this level of checking you might be financially better off using redundant cloud suppliers, say wasabi and B2, skip all of the checking, then when recovering data use wasabi as first choice because of zero egress costs, and B2 in case of problems with wasabi. Combined with your local backup, the odds on not being able to recover all of your data from the three sources is minimal. If you don’t have local backup, you should consider have two cloud backups in different regions.