I'd start by replacing your 13/23 cassette with a SHIMANO CS-HG200 8 speed 12-32 cassette (or something similar), a new chain and a proper size derailleur. This doesn't have to be an expensive change, and alone will reduce pedaling effort by 28% giving you an extra 2 - 3 lower gears.

if its the original chain rings, then it might be a good idea to do them too. A popular road bike double like a 50/34 might do the trick. Reducing pedaling effort by another 13% and insuring you drivetrain is up to spec. Aside from a new derailleur, which isn't expensive this is just normal maintenance.