docker-nginx VS vouch-proxy

Limiting an application can be interesting as some applications don't have support for automatic login via SSO, but you can certainly protect it. I have another image which is simply Nginx that allows yout o authenticate via Basic Auth against an LDAP server, or against your LemonLDAP:NG instance. (Single Sign on) At that point you would just proxy over to your application and after your session expired or if it was removed/disabled you would no longer be able to access those applications even if the account still existed on them due to the way the "handlers" work. All my other images work in the same fashion.

Look into vouch proxy

Vouch proxy is designed for this usage: https://github.com/vouch/vouch-proxy I don't think there are any nice UIs to configure it though so you'll need to be familiar with running it yourself.

Not sure this is a "best practice", but it lets me keep control of the Ingress resources inside their YAML configs. I've also layered Vouch Proxy into the ingress configurations to require SSO/MFA auth to access the resources behind the Ingress. Cloudflare has the ability to do this, but I found it cumbersome to keep track of the configs outside the K8s cluster.

I've used vouch proxy for my own stuff previously, before more recently moving to Cloudflare Access. vouch can be slightly janky at times to get working right, but once set up, it's been solid.

For example: nginx -> Vouch proxy -> KeyCloak -> Jellyfin

While this works, we were hoping to make access a bit easier with say an OpenID Connect SSO and reverse-proxy solution. I've seen Vouch Proxy, https://github.com/vouch/vouch-proxy which is really just SSO on top of nginx, but I'm wondering if there's a simpler way to do this.

You seem to be quite knowledgeable and a minimal provider with just the bare minimum would suffice for you. Have a look at Vouch Proxy, it does one thing and it does it well.