Edit: Noticed you asked for thigh high socks. These manufacturers probably don’t have any but I’ll leave the comment up in case someone else finds it useful.

If you’re asking for dress socks or something in that direction there are tons of brands.

On the “budget but quality” end I recommend Viccel although they’re from Turkey so not strictly EU.

https://www.viccelshop.com/about/

On the more expensive end of the spectrum there is:

https://www.bresciani.it/en

https://meschaussettesrouges.com/en

Then there’s Bexley, a cheap French store that used to sell shoes online and has since then expanded into other areas as well. Products weren’t necessarily top quality but they had a few notable shoe models that became popular for being decently priced/made back in the day. If you need “normally” priced socks that are over the calf they have them. They are made in China though so not exactly what we’re here for, but given the company is European maybe it’s an option if nothing else works out.

https://www.bexley.com/

And lastly there’s Suitsupply from the Netherlands (socks made in Italy). They at least sometimes sell over-the-calf socks but their socks and products in general were never really good quality and they’ve gone down hill from there over the years. They are cheap though and made in Europe so I thought I’d mention them too.

https://suitsupply.com/

Hope you find what you’re looking for!