Sorry for this late answer.
Thank you Orias for these orientations, and xM for analyse of these websites.
Doesn’t seem to be so obvious Jebba , or I’m really too too bad at sourcing exercise .
For M5 inserts I made a mod of y_mount_table in order to use washer+squared nut (a couple I’m also using in place of t-slot nuts, what is cheaper here).
Another solution for makers could be the use of Y_FrameConnector.stl in earlier TK-0 files. If I’ve well understood, the last design and use of M5 inserts, is to make easy for customer the mount of Y-frame with ZX-frame.
I’ve bought M3 inserts here (BW2388 M3 x 4mm straight knurl thru) : http://www.anchorinserts.co.uk .
It is not cheap (22,96€ / 120 pces + 20€ shipping) and I’m not sure about the final result …
I’ll try to stay as close as possible to original TAZ design for my first one. For my second I think about tryings mods in order to avoid such vitamins, hard to find, at least for me .
I’m not an ace for mods, but if interested I’ll share what I do here with pleasure.