I would try and dig it out and replace with an underground water tank of same dimensions. Although might be a problem for maintenance if under the slab.
Speak to your building inspector and see what they suggest also.
Ok so turns out our BSC got confused and said we needed 25 new piers. It should have been 25 total and we already had 21. So price is $1550. Thank God!
That’s good news. Maybe for extra peace of mind, ask what the price would be to
a) build solid slab over the area instead of waffle pod
b) cost to have a higher over all slab eg H1 instead of M class. The difference is they use more steel and better quality concrete which will make the slab more robust.
I would definately speak to the slab engineer and discuss these items and get their response. Also ask if they’ve had this on any other builds too.