Biohort Avantgarde with or without window?

I have another question about the base frame from Biohort

the cavity between the base frame and the patio slabs (foundation) should be sealed with a one-component sealant so that no water gets through?

Can you recommend an alternative to the original sealant (the original costs €30 per cartridge) you can probably use other manufacturers, right?

(...) Can you recommend an alternative to the original sealant (the original costs 30€ per cartridge)? Surely you can use other manufacturers?

You can. However, the manufacturer will then refuse any warranty for problems related to this.