I could be wrong, but I don't think there are many options for software that handles a large number of digits.

Perhaps someone that uses Lacerte knows how many digits it can use - that would be the easiest learning curve for you because Lacerte is extremely similar to PTO.

UltraTax MIGHT.  But I'm not sure.

I know that ProSystem fx does (and likely Axcess), but those are extremely expensive software options.