solution to the cost of actors is simple.
I think the issue is staffing, not actors. There was only one or two scare actors in the exit maze. But the experience used two staff to batch outside, another one to load into the "lifts", two staff to get everyone onto the ride section and then to shout everyone back out into the exit lifts. Then two staff on the baggage hold. Plus there may have been control room staff as well. Then add scare actors on top of the six staff minimum needed.
Compare this to Hex which has a similar three part experience, One person on the entry control panel and batching is counted by turnstyle. Two staff to load the ride and one in the control room. Total of four staff.
Yes the staff on Sub-Terra with the military inspired uniform add to the theme and it was designed that way, but it is a heavily staff led experience, even without scare actors.
I imagine (although I haven't bothered with Thorpe Park since it opened) that I'm a Celebrity suffers similar issues as it doesn't use "actors" but sounds like it needs a lot of staff to get guests in and out of each part of the experience.
Disney of course also use a lot of staff to move guests through various shows and attractions, but Disney I assume have accounted for that with costings. Merlin and/or AT park operations need to think those sort of things through at the design stage as to whether it is worthwhile having an attraction that needs a lot of staff to provide the experience.
I would expect that Nemesis the rollercoaster could probably operate with fewer staff than Sub-Terra could? Just due to the nature of the batching style.