Is there a European equivalent to SiriusXM or ADS-B weather in the cockpit, for display on an iPad through Garmin Pilot (or any other app?)
I’m from the USA, and I’ll be doing some flying in Europe this summer.
The short answer is Iridium data link based ones, requiring subscription and/or per-download payment. The only portable unit known to me is the Golze ones, which are often recommended on European forums such as this one. If the plane is equipped with a Garmin GSR56, then a Garmin Connext weather subscription will work, too. It is likely to be more expensive, though.
Depending on which corners of Europe you intend to go to, do check coverage of the various subproducts (radar, strikes, etc).
If you fly low enough, you’ll often have a mobile Internet connection, and can use that.
FYI, there are/were very localised ADS-B trials, privately funded. Not sure whether they are still running, but there was one in South England, and one in Denmark I believe. Garmin ran one last year in Friedrichshafen during the Aero Expo, I assume they turned it off afterwards.
As the others have posted above the GSR56 and the ADL devices are the main choices in Europe. But in your particular case I would suggest looking at the Iridium Go combined with the ADLConnect app which can also display the WX on ForeFlight:
In the long run the Iridium Go solution is more expensive to run than the ADL devices but in your case coming from the US the hardware is FCC certified and easily available in the US. Also it is popular among sailors (with other software) so you can probably resell it quite good on ebay if you no longer need it, whereas the ADL devices are popular in Europe but in the US probably impossible (and in some cases illegal due to FCC regulations) to resell.