Or search: astronomy, uranology, propagation
2 Syllables: SU
checklist, exit, fetish, friendship, headship, reddish, sexes, spendthrift, wreckage
3 Syllables: SUU
effective
3 Syllables: USU
affective, Alexis, collective, connective, corrective, deceptive, defective, detective, directive, elective, invective, objective, perceptive, perspective, prospective, protective, receptive, reflective, respective, selective, subjective
4 Syllables: UUSU
ineffective, introspective, irrespective, retrospective

Found 35 words across 4 groupings. Skipped 7 excess words of a similar stem.