[jyl]

I would switch cassette or freewheels, as applicable, to get the largest sprocket your derailleur will handle (look up the specs, maybe a 28). 42 x 28 is reasonably low for a road bike. Work on your legs and it should be low enough for 90% of climbs. I ride a 42 x 24.

On the high end, if you can get a 12 small cog that is 117 gear inches which is plenty high, the pro peloton is only a bit higher most days. Work on cadence so you can do 120 rpm which is 42 mph. No-one else you're riding with has a higher gear, so if you are being left beind on the decents either they are better at high rpm or at being aero.

If you want to go further - every bike part ever made is available on eBay, usually for much less than they cost new.

If you want to outfit that bike with Campagnolo 10 speed integrated brake/shifters, you can do so for $400-ish (Veloce) or $700-ish (used Record). Ditto a road triple if you have to have a stump pulling low and a super high. Just need to have the bike shop re-space the rear dropouts to 130mm and rebuild the rear wheel. It is very doable.