Most routers allow you to set child safety settings for devices to block them from accessing the internet in specific timeframes or completely. You can still access the local network from the affected device or access that device from your local network
Just constantly stick your finger up your nose, scratch your balls, etc. Make them dirty, greasy,... You get it. If there are "better" means (e.g. food) available, use that. Then play dumb and try to use their computer as if it was a phone and try to do everything as if it had a touch screen. Smear it all over their screen. Cuss as the PC doesn't react, try again, get visibly angry and irritated.
They'll stop asking questions fast. Probably right after they realize what you did to the first PC.
Daily incremental (and occasionally full) backup to an external HDD - a full image of my PCs, so that I should be able to restore anything back to what it was in the last ~14 days, assuming no ransomware or fire or...
All the data I care about gets synced to my Nextcloud (VPS, not home lab) - somewhat ransomware protected as I could restore VPS backups independently from my PC.
Most precious data (mostly photos) gets backed up regularly to an encrypted zip file and then gets send to a glacier tier S3 bucket. Some manual retention is done on the zip file level, so that I can get a tad older backup restored.
At least monthly a full backup image of my PCs is created on a separate external HDD which is not stored at home, but in a place I could access 24/7 if I really needed to restore something fast.
Phones, etc? Just sync to the mentioned Nextcloud, PC downloads from there and everything gets then into the aforementioned backups.
Homeserver? See "PC" above. With the caveat that some VMs/containers are not in the backup cycle, as they do not store any valuable data besides temp files, etc. For these, only things like docker compose files, custom config, ansible playbooks,... are in my backup.
Somewhat, I read a German news article that explicitly warned that USB transfer will be blocked. I just searched for an English article to post here afterwards, but I didn’t read it. So… yeah, „lost in translation“
So I'd suggest, unless you really really need some obscure feature, Calibre+Kindle is nowadays perfectly fine, and maybe you shouldn't risk bricking your device.
Uhm... Well about that... You will not be able to transfer books onto you kindle via USB in about a week. Amazon is going to remove that feature from all Kindles next week. The only way to do that may be through the method you described. But how long will they offer that, if they say they are removing the USB feature because of piracy? You cannot pirate books onto your kindle, when you cannot transfer books from outside of Amazon onto it. (Also this is a nice reason for them to block you from buying books anywhere else than on Amazon, of course)
I don’t mean that you need to enter info there that you don’t have. But - you found a new shop? Cool. Enter it in OSM. Or checking it’s correctly in OSM. Or append their opening hours to the data set or…
For some easy edits, there’s e.g. the android(?) app StreetComplete which will show „quests“ on OSM in your area to complete datasets. Like „is this street lit?“ „what surface does this road have?“, „are there board walks?“, „what’s this house number here?“, „what’s name of this street?“ and so on.
You can do such things on an afternoon stroll in your area and it will help you learn your surroundings and discover things.
Deutsche Bahn will definitely proof that public transit in the EU isn’t necessarily…. there? Working? …