It’s worth noting that co2 is allowed in Europe. Specifically upto 4 canisters upto 28g each.
You should take them carry-on to avoid getting your suitcase opened and to give you a chance to explain their use.
I get stopped maybe 50% of the time with co2 canisters, then it’s generally just a case of showing them the regs or waiting for them to go check themselves. I’ve not had them taken off me yet.
Whether it’s worth it to avoid just buying some more at the other end is another question. For me, it’s the convenience of not having to go buy some, but if you’re renting a bike it’ll either include them or they’ll sell you some.