You seem to have a cheap tracker at around 2% interest.
Who is this mortgage with?
Would they do a deal with you something along the following lines
Sell the apartment which will leave you with a €70k shortfall.
Pay €35k off the shortfall to bring it down to €35k.
Add that to a new SVR mortgage.
The lender might go for this if it's a live lender.
If it's a dead lender like BoSI or Danske Bank, you should wait and hope that they might do a deal for the early repayment.
There is no harm in continuing to rent as you retain an interest in the property market with your apartment.
Don't forget that your apartment is a profiable investment in that the rent of €5,000 a year exceeds the interest cost of €2,000 a year. You are knocking €3,000 a year off the capital. It might not seem a lot, but the €70k negative equity will look much smaller after a few years.