The best solution: take the equity and borrow the rest privately from family.
That's how I would do it too. The missing money can be scraped together somehow.
If necessary, also consider selling long-unused valuables... old jewelry and such, there will be no better incentive to part with the dust collectors. Some people also have a €2000 bike in the basement and haven't ridden it in a year.
Ask family, take out several small online loans and then you'll have the money quickly together and easily paid back in 2 years.
For example, save the relatively expensive mortgage registration in the land register/notary.