Estate Filing 706-no 706 available

Unfortunately, I think you already answered your own question.

"and really didn't want to do a paper extension or manual return"

I'm in the same situation -- haven't done a 706 for several years, now have two I would like to finish before tax season.  A couple thoughts:

1) There is a company that sells the 2024 software for $170.  Assuming your client estate is worth several million, is that too much to ask them to pay for your convenience?  Add in more, of course, for installing and learning the program's quirks.

2)  Are the schedules and forms any different from 2023 to 2024?  Is the only change, the first couple pages where the computations are done?  Can you use the software for everything but those, and then do those manually?  It has to be a paper, mailed-in return anyway.  

I believe the IRS has not released the final instructions for 2024. Hopefully when they do, Intuit will provide the 706 soon after.

If you're only filing for portability, there isn't any tax due so you don't need software to calculate anything. Do a paper extension.

Or note the special rule for this situation that gives extra time to file.

I'm doing paper extensions for federal for this situation, and MD where there is tax due.