Gardener only mentions net prices

yes it does. Including VAT.

yes, it does. Including VAT.

Ok! And then he mentioned prices for the additional works that were net amounts and now logically appear with VAT in the final invoice? What did he say when you asked whether it was net or gross?

The base offer would therefore be correct, and it’s about the additionally commissioned items, right?

there are enough rulings where private individuals were misled with net prices.

Example
Court Rules in Favor of Association: Net Prices Not Allowed
The Heidelberg Regional Court sided with consumer protection organizations (judgment dated 12.08.2016, file no. 3 O 149/16). The court ruled that moving companies were not allowed to use net prices towards consumers. The T&Cs clause was therefore inadmissible and invalid.

Even on MyHammer, there is an article stating that private individuals must be given gross prices; if this is not the case, there is a fine of up to 25,000 euros.

I would let it come to that

I would actually be satisfied with the solution that he trims/prunes the plants and hedges for me for the next x years. He’s always in the area anyway, so at least he saves himself the trip and I don’t have any effort ;)

He'll just say in court if necessary that he verbally warned you, and then it's his word against yours.

I would either insist that he simply made a mistake for which you are not responsible, and if necessary take it to the limit, or agree with him now (50/50, whatever). The good man might die next week from Corona, fall off a ladder, go bankrupt, close his business, etc., and you'll be left empty-handed again. So it's better to settle and close the matter now, no matter what.

I would make it dependent on whether he basically does (craftsman) good work and you are satisfied. Then you can make him such an offer and point out that there are certainly more jobs or that this amount could also be offset with them. If things went so-so or not everything was done cleanly, transfer the invoice net amount as gross (so correct from your point of view, reduced from his point of view). And just wait. Then he has to prove without doubt that he named the prices to you correctly....