Or search: acrobatic, activated, activist, agile, astir, astronomy, athletic, brisk, bustling, dynamic, energetic, eruptive, existent, grammar, gymnastic
2 Syllables: SU
abbess, agnes, atkins, axes, bacchus, baggage, baptist, blackish, blacksmith, brackish, cabbage, captive, happenings, lapses, napkins, package, papists, practice, practise, radcliffe, satis, taxes, trackless, waxes
3 Syllables: USU
adaptive, attractive, collapses, combative, establish, inactive, reactive, refractive, relapses, relaxes
4 Syllables: UUSU
interactive, reestablish, unattractive
5 Syllables: UUUSU
radioactive

Found 38 words across 4 groupings. Skipped 7 excess words of a similar stem.