Usually, the current market value is quite a bit above the BRW. For me, the current BRW would rather be a negotiation offer. What you have paid so far were more like rental interest and not an interest-free down payment.
I think so too: leasehold is not rent-to-own.
Unfortunately, we do not know how high the market value of a property encumbered with leasehold is.
But you do know what the market value is for roughly similar plots for sale currently in your area.
You would have to start there first. Then possibly mediate with the amount still to be paid...
Market value €300,000
Minus still to be paid €75,000
€225,000 and then into mediation, I estimate between €150,000 and €200,000