I had no problems using my A1 B.free Internet SIM in Austria in June. I don't recall trying to do tethering in this trip but I know it's worked in the past.
I add credit using
https://www.onlineaufladen.at/?lang=eng. It works fine with a Canadian credit card from within the EU. However they require that the card is registered with Verified by Visa.
They also use geolocation to block users from North America. However I have circumvented that restriction in the past using a free VPN service, e.g.
TunnelBear.
I ordered this card online from Canada several years ago and had them send it to my hotel in Austria. There was no problem with that but that was quite a while ago. I've since added credit to this SIM every year, usually online from Canada or Austria. I've also bought vouchers at A1 shops. Other than the issues above the SIM itself has worked flawlessly.
I'll be back in Austria in a couple of weeks. I'm looking forward to trying out the new EU-wide surcharge-free roaming from Munich airport.
As you said, it's also possible to buy this SIM at grocery stores and post offices. I've never done that but I don't see why there would be any problems. If you plan to do this you might want to ask your hotel keeper for advice/help. Unlike in Germany you don't need to be a resident, show ID, etc. It should be a simple transaction. I don't recall what I had to do to activate the SIM but the hotel should be able to assist if you don't know German.
You can monitor your data usage using either A1's website or their Mein A1 app. Again it's all in German, except for this summary:
https://www.a1.net/bfree-prepaid-local-sim. BTW the Mein A1 app can't be downloaded from the Google app store in North America. But you can download it and sideload install from
https://mein-a1.en.aptoide.com/
Have a safe trip. What village are you going to visit?