Do a temperature test with your hand while the system is operating. You may be able to find the problem that way.

Also, I believe the expansion valve, while it may be behind the firewall is accessible at the firewall.

I may be wrong on that. I’ve been retired for three years and I forgotten more than I remember. And yes, go to a shop to recover the refrigerant in the system and find out actually how much refrigerant was in the system. Then after repairs are made, evacuate/vacuum at least 40 minutes, an hour is better. And be sure to add a little oil to the recharge to save your compressor.