And keycloak has a decent k8s operator, making deployment on a k8s cluster a breeze

accessed from the internet

Accessed only by you and close family/friends who you are also hosting services for?
Or accessed by anyone?

“Accessed by anyone” carries more risk.

“Accessed by users you host for”, the risks can be eliminated (well, other than risks from those users) by using a VPN. As in, only the people authorised to be on the VPN can access the services.
Wireguard is the go-to these days.
Tailscale is much easier and free for 3 users and 100 nodes.

If it absolutely has to be “accessed by anyone” I would look into a “reverse proxy over VPN/tunnel” or just straight tunnel style approach like chisel (or crowbar, or corkscrew), rathole, frp, or cloudflare tunnels.

Basically, don’t point a domain at your home public IP and don’t forward ports on your home router/firewall

It’s a server with integrated UPS and KVM console.

So you have local DNS set up?
If you ping (or dig) speed.mydomain.local, does it resolve the same address as local_ip?
Considering you are accessing local_ip:3000 and the domain on port 443, there is clearly a firewall somewhere redirecting packets or a reverse proxy on the domain but not on local_ip:3000

Follow the port chain, forwarding, proxying etc. One of those will be bottlenecking. Then figure out why

Edit:
Just because your ISP speed is 100mbps and you are seeing 500mbps, doesn’t mean the connection isn’t hairpinning through your router via it’s public IP (as in, the traffic never leaves your router, but still goes through it)