Need to connect new 3" ABS pipe to an old cast iron cleanout pipe just above cement floor. Need to create a new cleanout in 3" ABS that will be 12" above the old cast iron cleanout so that a main sewer line blockage between the main stack & the street sewer some 25 feet away can be removed. Had to remove a basement toilet that was installed on a 12" platform & connected directly into this same old cast iron cleanout with 3" ABS a long time ago. Tried a 25 foot snake through the toilet bowl a few times, with no luck! Before reinstalling the toilet, after removing the sewer blockage, need to make a sensible remedy that allows easy access to the main sewer line if there is any future blockage. Then a Root Removal product can be added regulary to prevent further blockage since the sewer line travels through garden space that I'm not willing to give up. (There's been no blockage in this sewer line for more than 15 years.)Was recommended a product called "PC 4" as a bonding agent for ABS to iron. Can you suggest anything?

We don't use ABS in my area let me talk you into using PVC. To couple 3" cast Iron you will need a 3" PVC to cast iron No-Hub Band,(see image) Also a 3" clean out tee can be glued on to your stack for easy access of a sewer snake. If you wish to control roots RootX or Robics Foaming Root Killer that contain Dichlobrnic are two products you could try.
A less expensive way would be to call around to garden supply stores and ask for fine grain Copper Sulphate. Put a 1/2 pound in your toilet and flush it down. Repeat in 6 months.
Hope this helps and thank you for rating my answer, Tomoots