We find sleepers can need replacing in 5 years, 25 years, or even 50 years, and not a clue why some rot and others don't, all ours covered in coke oven waste, which clearly no longer available as no coke ovens any more. So moving to plastic, they say they will last 100 years, once changed we use one of these

to level them up, so swapping sleepers is an expensive job, so worth the extra so it will not need doing again.
You have the same problem, to remove them means digging around them to re-lay them, it is the groundwork which costs, the sleepers compared with work involved are cheap.
I would have used crib lock or something similar it was made by Ruthin pre-cast, but I know we used a fibre mat and old sleepers at Sylfean passing loop/station which also holds back a bank of earth, done during the winter months, it took around 2 months or more with volunteer labour, if one was to look as cost using paid labour it would be massive.

OK, this needs to take the weight of a train, but not that high

far side of track shown here down to farmers field, the cost of hiring rollers etc, must have been huge, but now we can run 4 trains at the same time, don't think ever done, as we don't have 4 engines in steam.
But you are looking at many £1000s of pounds, and not really a DIY job unless you have loads of people involved.