I have no idea what the situation is with availability of insulation contractors and supplies, but I'm guessing years? Certainly won't be possible by this winter.

You might get 50% on a leaky barn with zero insulation, but how many homes in Europe are like that?

(Note I'm not saying not to do this - certainly do this, just have proper expectations.)

Such properties are common over Europe; especially in properties built before the 1980s (which thank to the rebuild after WW2 are a decent percentage of all building stock).

Overall you'll maybe cut gas usage by 20% in 2-3 of years by insulating those buildings, building out renewable electricity with storage and incentivising heat pumps (which reduce/remove gas usage of resident-owned properties where they are viable).

Add-on insulation can help but it’s much harder and not as great.

I'll note that it's not always worth it to further insulate or replace functional though inefficient equipment. For example my apartment has an electric bill of about $50 a month, half of that is a fixed infrastructure cost. Even if I could bring my electric use to zero, it could at best save $25 a month. Replacing my AC with a more modern one could probably at best save $5 a month of that, taking many years to pay off even if you pay nothing for labor.