Newly built first-time horror condominium purchased

Tax is only due if there is actually a profit. The OP apparently lives in a situation that is untenable for him; do we really want to discuss tax avoidance in this context?

Really bad idea, because speculation tax applies!!! Consider this beforehand and then decide finally before you get annoyed with the tax office.

That's true, speculation tax is an issue in this context. There seems to be an exception if the property was owner-occupied in the year of sale and the two preceding years. But it is indeed a matter for a specialist.

that is a property previously used personally - no capital gains tax applies!

Whether to sell or continue renting can still be decided then, or the tenant may buy the apartment from you at a good price.

Very bad idea, because capital gains tax applies!!! Consider this beforehand and then make a final decision before you get annoyed with the tax office.

This is a property that has been owner-occupied until now – no capital gains tax applies!

Do I understand your words in combination correctly that the OP would now sell owner-occupied without capital gains tax before sale, but with interim renting would either have to use it owner-occupied again for one year before selling or wait until he has owned the apartment for ten years?

Do I understand your words in combination correctly, that the original poster would now sell the property self-used without speculation tax before the sale, but in the case of interim rental, would have to either use it themselves again for one year before a sale or wait until they have owned the apartment for ten years?

Almost. After rental, he would have to have used the property himself in the year of sale and the two years before.
Or hold the property for a total of 10 years.

Or thirdly: if the property was used exclusively for own use. That means since purchase no one other than the buyer, their partner and/or their children were registered there until sale.

In any case, I understand § 23 Income Tax Act, paragraph 1 as follows

“Excepted are assets that were used exclusively for own residential purposes or in the year of sale and the two preceding years for own residential purposes between acquisition or completion and sale;”