@dub lad
TTS3 first appeared in the final nuvi range (2xx7/3797 in Mar/Apr 2013). So, your zumo 595 released in April 2016 wasn't one of the first devices to use TTS3, aka "Real Voice", VPM files which give landmark names in navigation directions and is still used by recently released Garmin PNDs. Unfortunately, turboccc seems to have retired from Garmin modding many years ago, so TTS Voice Editor is orphaned. It remains relevant to edit TTS(1) and TTS2 files but can't cope with TTS3 as you found.

The RU method in Post #3 of [Only registered and activated users can see links. ] is indeed relevant. Member osiris4isis explains it further in posts on Page 2 there. Unfortunately, he also no longer seems to visit here either.

Good luck, but if you do soft-brick your device with a faulty VPM file we can easily help you cure it so don't worry too much about "destroying something".

Please post back and let us know how it goes, all feedback on such experimentation is welcome here - even if it's a negative outcome.