Part of the reason it says unsecure is because the certificate that the site is using isn’t for your ip address it’s for “myfortressofsolitude.duckdns.org”, this causes the browser to say the connection is not secure because it cannot validate the connection against the certificate. To prove this click on the lock icon beside the url and have a look at the messaging inside there of it, which should say Certificate invalid
If you do the same with your other url it will say Certificate valid.
I use SSL connections without using the default port personally and never have issues with not having a secure connection unless I use an IP Address which is as I stated because my cert is not for that web address.
Nginx is a great product and I do not disagree with the use of it if your use case calls for using that particular tool. I am using the duckdns addon without Nginx and a non standard SSL port. Neither the port of 443/8443 nor nginx is required to make your connection communicate over ssl. If you are attempting to use a port proxy service to expose to the outside world that is great for that but not required.
I would expect that is observably faster than 443, if 8123 is going directly into your home assistant instance, this is because with Nginx in the middle of your trip from your computer you have now intentionally added an additional hop to reach your final destination If your nginx server is also on the same pi as the home assistant then the resources are being taken away from home assistant as the network traffic occurs. This would cause an observable slow down when using that particular port of 443.
I feel like I have derailed the particular thread and I am willing to talk more about this in a different thread. As long as @Andre_Bruno has had his issue resolved by any method he could I am happy.