I'd keep Sinistcha. Rage Powder gives Tyranitar a chance to Dragon Dance, and Hospitality helps your attackers stay useful when you reposition. I'd mainly use its Trick Room to reverse an opponent's room, rather than turn it on alongside Sand Rush Excadrill or Unburden Sneasler.
For the sixth slot, I'd test Competitive Milotic. Javier Varela won Gdańsk with Tyranitar / Excadrill / Sinistcha / Milotic as part of his six, so there's a recent regional example of that support structure working. His Milotic had Scald / Ice Beam / Icy Wind / Protect with Sitrus Berry. To try that here without duplicate items, move Sinistcha to Rocky Helmet; you lose its immediate healing but gain contact chip. Milotic gives you another special attacker and makes opposing Intimidate less comfortable.
Two small fixes before changing the core: Rock Slide over Earthquake on Excadrill, since all four current partners are grounded, and Shadow Ball over Protect on Scarf Indeedee. Protect can lock Indeedee into a very passive turn before it has used Trick. Keep High Horsepower for safe Ground damage beside your own team.