I would definitely not buy, or rather, the purchase of the plot was also delayed for us and we did not buy either... If you use the apartment as equity, the bank will surely want to register itself in the land register there as well. If you later want to sell the apartment, the first problems will already arise.
To obtain tax advantages, one should always try to use the equity for the owner-occupied property and acquire the rental property with the highest possible loan portion (in the case of a house with a granny flat, this is often done with 2 separate loans). Besides, you simply have to see that a condominium is always also a certain speculative object with certain risks. If you have a problem tenant or there is a large construction site nearby, this always affects the rent and ultimately your own home as well.
Better leave it be. As it sounds, it’s no longer about 5-10 years until the plot purchase.