TUI have operated at STN with 2 aircraft I think going all the way back to 2009. Before that it was First Choice Airways. I'm not sure Thomsonfly as was had a base at STN, only the odd W-pattern, but Britannia had a couple of 757's based prior to that. The regular long-haul programme only operated for Summer 2016 and 17.
Until Covid at least, TUI's growth at most regional UK bases like CWL, EMA, NCL, previously DSA, had been still fairly modest but all out-paced STN.
The clear change to the STN offering at least has been a reduction in destinations offered but better frequencies on those that have remained to allow 10 & 11 night durations to package holidays. Other than SSH (no competition) every destination is served at least 2x weekly during the summer. Until 2019 they were still serving I think BOJ, EFL, FAO, KGS, LCA, LPA, TFS and ZTH all with just 1 weekly flight.
STN would seem a prime place for at least a third aircraft to re-instate some of those destinations alongside the better frequencies. Whilst they've reduced LTN to just 1 aircraft it's clear their primary focus is LGW. That said they sell a lot of packages using EasyJet flights, so their package holidays offering from STN is comparitively limited.
As for long-haul, they've probably got about the right balance now, but EMA first, then probably STN would seem best placed for those.