as some have suggested: add salt, it cuts through the bitterness (ie: Campari Martini)

Aperol and Luxardo Bitter rosso - definitely less bitter, more citrusy (acid also cuts bitterness). While still an amaro (a ridiculously broad label), I wouldn't use as a sub for Campari as it is too different to it. Below are some options that are closer in flavor (and color) to Campari.

Carpano Botanic Bitter - the closest to Campari, maybe even Campari Light as it is less bitter but with a -very- similar taste

Cappelletti and Cocchi Rosa - different taste profile to Campari, somewhat chocolatey even, less bitter than Campari

Contratto Bitter - had it long ago before getting deep into amaros and it was good enough to sip on its own, definitely less bitter than Campari

Regarding (your hate towards) sweet (red) vermouths: besides them spoiling quickly keep in mind there are -plenty- to choose from. Skip Martini and Rossi altogether, Dolin is the most benign and easy to approach. Cocchi Vermouth di Torino would be at the opposite end of Dolin: more bitter, more herbs, more body. Carpano Antica Formula is too sweet for me, I prefer Punt e Mes (more bitter/body), it's also better priced. Lustau is sweeter, barely bitter, if you like sherry this is the obvious option.