I've had good success spinning up an authentik instance, and having my reverse proxy hit it first before it routes to the actual app. Puts another security layer in front in case sonarr itself has an issue, for ex.

Caddy snippet

handle {$host} {
		route {
			# always forward outpost path to actual outpost
			reverse_proxy /outpost.goauthentik.io/* authentik-server-1:9000

			# forward authentication to outpost
			forward_auth authentik-server-1:9000 {
				uri /outpost.goauthentik.io/auth/caddy

				# capitalization of the headers is important, otherwise they will be empty
				copy_headers X-Authentik-Username X-Authentik-Groups X-Authentik-Entitlements X-Authentik-Email X-Authentik-Name X-Authentik-Uid X-Authentik-Jwt X-Authentik-Meta-Jwks X-Authentik-Meta-Outpost X-Authentik-Meta-Provider X-Authentik-Meta-App X-Authentik-Meta-Version
			}
			
			reverse_proxy {this_host_or_ip}:{this_port}
		}
}

Not gonna solve all your problems. Works for 70% of apps. Nice to slap on when you can.