I was curious after reading your post about sub $10k full size TLB units so I checked CL for my area and surrounding states. The used units are considerably more expensive than $10K. 3 to 6 times more expensive.
Seems prices have gone up and I have not looked much since I bought the Ford 550, but looking "today" at older TLB's locally just on CL heavy equipment:
Even the 1971 Deere would put a new SCUT to shame digging 6 foot deep of course jmo
$7500
$12.5K
really old running JD with a home built trailer asking
$16.9 Case 580 C Runs great
Second to last trench I did with the 550 36" deep X 400' It was a little hard on the Harbor freight 12" wide bucket I adapted instead a full 2 foot wide trench, but some welder work fixed it up. My guess is that 100lb HF bucket is probably about as beefy as what come on a new SCUT backhoe for a bucket.
TLB can make short work of moving a full 3300lb pallet of concrete if needed for a job as well. New is always nice but there is serviceable old equipment out there, many people buy an old machine get the jobs done and then sell them for close to what they paid for them. I just truly believe a newish/new SCUT would loose against even the lowest priced backhoe above digging the depths the OP is asking about especially if the SCUT or old backhoe is also tasked with transporting those spoils.
If I was to buy today out of the list above the Case 580 C would be it, Even at the 16.9 K price, if as described should also be powered by the venerable Cummins 4BT and has modular drivetrain parts replacement unlike a Ford 550-555. Oh and if a guy feels like it with some of the money saved add a hydraulic thumb to the backhoe.
After thinking about this some more maybe the Op should just rent a nice Kubota KX040 or something similar and then buy a CUT/SCUT that fits all the other more tractor only uses the OP may have planned.