Not sure that is any different then an other PDF file when opening it in illustrator and having to deal with fonts not embedded or you don't have. We ways open the PDF in Acrobat and convert text to outline before opening in Illustrator.
Well bad news seems to be coming your way as well as the way of all the rest of us. Like you guys I grapple with customer's home baked files and one of my go-to's for missing fonts was Acrobat's 'print production | pre-flight | convert fonts to outlines'. Except then it seemed to stop working.
One option I used to use to get around this if I didn't need to open and edit the offending file was to place it onto the Illustrator art board, add a die line, step and repeat and then save the file to load into the RIP. Since Illustrator 2025 (29.5.x) a warning comes up when opening that there are missing fonts and again when saving a warning pops up that there are missing fonts which won't be included in the saved file so after saving the file and opening it in the RIP indeed the text is missing.
So falling back to Acrobat's 'print production | pre-flight | convert fonts to outlines' the fonts were reported as being successfully converted. However opening the file in Illustrator 2025 shows no outlines and only missing fonts. I thought Acrobat Pro was at fault until I found when opening files that had been processed with Acrobat's 'print production | pre-flight | convert fonts to outlines' in Illustrator 2023 (I think 2024 is also ok) did show the outlines were present. Yet opening the same file in Illustrator 2025 again there were no outlines.
Then a designer sent me some pdf's and he works with layers where he has one layer as text and another layer as outlines. He turns the text layer off and sends me the pdf, but Illustrator 2025 only shows missing fonts without any outlines so now he has to delete the text layer before sending me the file.
Clearly all aimed at screwing font licensing down but personally I find it way OTT.
Due to using some third party extensions in Illustrator that are always late being updated to compatibility with Illustrator'sl atest update I always run three versions of Illustrator being current (for future use) plus two releases prior. Now what I do is run the Acrobat's print production | pre-flight | convert fonts to outlines, open the file into Illustrator 2023, save it out and then reopen it in Illustrator 2025 with no problems (for the moment, until Adobe tightens the screw a few more turns).
Another aspect in all of this is that from Creative Cloud you can now only access the current version of Illustrator plus one version prior with anything older having been removed so if you want earlier versions you have to contact Adobe and explain yourself. Not sure that "... so that I can get around your onerous Illustrator 2025 fonts licensing" is going to cut the mustard. Also from around the time older software versions were removed from Creative Cloud Adobe's licensing now appears to only authorize you to use the current plus one prior version, the right to use older versions appears to be rescinded.
I do use other processes to work around the outlines issue but the time it takes all starts adding up and customer's files keep getting worse, the world's just getting more and more complicated.