So do you not keep covering your tattie plants as they grow?

....sure thing! That's the 'earthing up' process and it's far from nonsense lol. When I was in Portugal, with the soil so rich and fertile, it wasn't done coz the tatties were able to find themselves enough room underground to spread. Here in UK it's pretty common though, and where I am the ground has a lot of clay and is dense. The earthing up (I did it 2-3 times) means you can have a couple of layers of tatties rather than just a flat spread.

I reckon your grow bags should work just fine - a bit of earth on the bottom followed by seed tattie(s). Cover them with earth and let the shoots get a few inches high before covering again (and then again once they are another few inches, until you get to the top). It should work as well as the open ground IMO :)

one of the tattie fields in Portugal

Thanks so much for your interest Sam, here's an old blog index in case you'd like to explore. It's from before I met @flamistan lol.
🤗🙏