I have a Diamond BAck Fleetstreak from that era ---- its a fun bike

I don't think its financially prudent to try to sink a lot of money into the beast --- It already has a triple chainring drivetrain , which is a huge help on the hills ---- for making it more commuter friendly , I would install some semi-slick tires to minimize rolling resistance , then call it a day

Spending too much money trying to make it lighter or faster is a bit like installing hot rod parts in a Bronco or a Jeep --- at the end of the day, its still a Bronco, know what I mean?

The best upgrade for a commiter would be to keep your eye out for a secondhand touring bike, or a road bike with some tire clearance for racks, fenders and the like -- maybe an old Cannondale or Trek touring machine, Specialized Sequoia , there are a bunch of Miyata touring models - etc.