SD cards aren’t reliable.
Although it’s better to use them for backups than for the installation, you are still better of with an SSD.
Or some other medium for backups (cloud/network).
The backup is supposed to save you, not fail you when you need it.
The backup needs to be in a format you can read it away from the machine in case the machine itself dies. Plus if you need to scratch reload you will need the backup to install whatever tricks you are going to use to re-mound the SD card. So No, I suggest against that.
Adding an internal mounted drive is not as easy as it sounds if using HAOS anyway.
Size. I would allow for 12 writes before filling so I would get at least 12x HA backup size. SD writes is the issue and if your constantly overwriting I think that would be bad so presuming 1 backup per month 12x is recommendation.
Quality. Dont cheap out. Get something from quality vendor that has some measured quality. I think Samsung is something like sdxc or similar
SD card had limited write qty and for this reason it has burned many HA user after the database killed there sd cards running in their RasPi. With proper care and consideration SD card could be Ok.
Is this HAOS? I don’t use HAOS but maybe udev rules are correct way to mount sd. If not maybe it should be added as media folder somehow.
And there are plenty of cards that are fake sold for the full price.
I strongly advice against using a medium prone to fail for backups.
The best option is cloud based if you trust Google. I find it very unlikely they will screw up with their backups.
What are you backing up. Only care about config which is fairly static data and unchanging DB data i don’t currently use. I agree, if data is super important your backup method and medium should reflect that. I’m not putting family photos on SD
Full backups every morning.
I don’t see any reason to do partial backups.
The data itself is not important, but automations, config and all that.
If I would backup once a month then it would be a lot that could be lost.
Sometimes you create a really complex automation/Yaml/Jinja and I would not want to loose that just because I back up once a month.
So your tool can help you with what?
Tell you that you already paid for something that is fake?
With some bad luck then the seller won’t care because you have opened the package.