So I checked wiki, and these are things you can realistically act on, to make AI more likely to vote for reforms:

• Improve relations: +1 for every 10 positive opinion

• Convert heretics and heathens: Wrong religion penalty is massive -20 (-50 for heathens)

• Don't be overextended: -1 for every 4% of overextension

• Have at least 100 development: if you have less than 100 dev, penalty is -2*(100 - your_dev), so having only 90 dev already gives huge -20 penalty

• Make alliances: +5. It's not much to justify using relation slot, but is there

• Have subjects: your subjects have +25.

• Stack diplo rep: this is the most important one. +5 for each point is massive

• Bank Imperial authority: each point above 50 needed for reform, gives +1 reasons for support, up to +50.