Thanks for the hint I did not know about Nabu Casa.

Although I am not clear about the benefit of the service. I thought I could achieve the same kind of access with simply forwarding ports 80 and 443 from my router to my home assistant. Is there anything wrong with my intended setup?

Using a VPN does work for restricting outside access, however I found that for me this was not convenient enough for two reasons:

1. I would need to setup a VPN server at my home network
2. I would need to connect to my VPN everytime before trying to access my smart home devices

Since I am on the same journey as OP and tried it just recently, I have one thing to add about openHAB:

It does not come with actual login credential handling. If you want to make your smart home accessible from outside your home network I cannot recommend openHAB.

I am currently going with Home Aisstant mainly for that reason.