My Hannigan conversion has a true IRS, with CV joints. Hannigan has been using pretty much the same components, for almost 40yrs, with excellent reliability. Issues with their suspension suspension is rare.

Whatever the cost to convert a TriGlide to IRS, it wouldn't be worth it to me. What I do know is that among trike conversion manufacturers that offer either solid axle or IRS, the latter option is usually around $2K or less.
Others and myself have already covered the pros and cons between a factory trike and conversions so there's no sense in beating that dead horse, again.
Enjoy the ride, that's all that's important.