I live in a city that has a centralised heating infrastructure (every building gets a hot water connection), and my apartment came equipped with a heat exchanger that connects to said heating system. I have to _lease_ this heat exchanger from the energy company, and I cannot opt out. The lease is like 50 euros per month, though the MSRP on the unit is only 400 euros or so. Then I still need to pay for actual heat consumption, which is also 3 times more expensive than a gas powered heater that is commonly found in my country.

We don't do per-apartment heat exchangers, but rather one per building, typically with shared hot water radiators across all units in the building. Heat consumption is either estimated by a air/radiator temperature difference integrator on the radiator, or an electronic flow meter+temperature differential sensors on the unit's connection to the central hot water loop.

Always look into buyout terms. Though replacement heaters these days are lot more expensive than pre-covid :(

If so, I suggest installing a water softener with an anti-backflow device but not installing a bladder pressure relief tank between it and the water heater. Also buy a whole house leak detector / shutoff valve.

If the resulting water heater tank rupture doesn't lead them to breach the maintenance side of the contract, look into over-softening the water so the sacrificial anode fails every 6-12 months.

(Edit: In case it wasn't clear, this is a great way to do unbounded amounts of damage to your house, so don't actually do this.)