I know that now with the Operator:Min + Trick:Die I can stop circle spell bullets before the 20th cast but there is no way to make a circle spell last longer, if I'm not mistaken.
Can you make a Trick that take a Constant Number and at that loopcast index it will terminate itself and to balance it, if the constant is bigger then 20 then it Cost more, or just make it linear all the way down.

No.

That was quick :D