Glad you are getting a 2nd opinion.
Shimano RSX 8 speed is fairly compatible with many of Shimano road groupset parts (10 speed and under) and mountain (9 speed and under), with a few exceptions.
Hopefully this shop will give you a full evaluation and a plan to move forward. Not sure what their availability of new parts are right now with the current supply chain issues. If they/you can get it running for now at a satisfactory level, a slow upgrade can be done in the future as parts are available, either new or from the used market.
Let us know what you find out and post any questions. Many members have worked in similar bikes and components.