... I've rebedded all my stanchions, pulpit and pushpit (incl. studs!) and all six mooring cleats and that stopped the toe rail from leaking. ...
Did you do it properly; removing the deck fitting and bedding above and below, or just above? I've done both, but my main problem was wobble.
Yes, removed the whole stud (photo). The "backing plate" for them is just a washer underneath the nut, so no big surprise they wobble. I've found they wobbled more with butyl tape between stud and GRP (white ring in the hole photo, with countersunk hole in center), so on the later ones I omitted that and instead sealed only the sides and top of the stud (between stud and stanchion), which is still leak-free but less wobbly. I then put a bigger (penny) washer on the stud and that helped some more. The remaining wobble originates from the stanchion tube being slightly larger than the stud - not much to be done about that.
I've learnt to appreciate the wobble
Keeps visiting crew from using the things as handholds (there are plenty of better, sturdier and safer handholds inboard on the coachroof).