Just tax CO2 emissions to a sufficient level and let the market sort itself out.

Yes and in this case this could achieved very easily by simply increasing taxes on kerosene, in a way that does not affect existing and future 'green' aviation fuels.

That would kill what Google Flights call Hacker Fare (& many others too before Google). Essentially you buy a local leg ticket, & another leg ticket, international in your example. Both airlines does not know about other legs (although thus not responsible for any delay or missing nezt leg).

What would stop people from doing split tickets?

The above idea specifically discourages split tickets (buying one ticket for each leg). You'd end up with an extra 70€ fee if you buy the two tickets separately. So unless buying a multi-leg tickets adds more than 70€ to the total price it's not worth the trouble.

If you split it you'd pay more, no?

Yeah, I don't know how that never occurred to me.