Is there a way to optimize for smooth gaming?

Without proper measurement / data nobody can help you. Like:

  • ping from your PC to 8.8.8.8 or 1.1.1.1 or some other fast sites
  • ping from your friend’s PC ?
  • ping between you too through ZT ?

I recommend to use 9ping, which helps to see clear data, running for 5-50 minutes.

Also maybe read my answer in the other topic:

But that costs money. (Self host a 1core VPS in your own country = $5 / month)

If ZT does not work for you, you may try other VPN systems, like:
WireGuard, SoftEtherVPN, etc…