I believe I am good with tools, good with engines, family mechanics. I am mid of two gearbox rebuilds (early and late boxes, new synchros, new lsd, bits and pieces), however the work is being done by someone who I hold very high as to having a professional level of knowledge, experience and skill - who also has some of the specialist tools (custom made). Forever grateful.
Personally a professional is worth the expense/importance of a good gearbox.

With the gearbox out you might be able to inspect any broken teeth on the internal ring.

Why I am posting... Remind me / Pm me if I don’t repost, I have both mechanical and speedo drives (different ends from memory), I have both blue (early model box) and white (late model box) speedo rings (from memory they were different diameters/sat on the final drive differently so are not interchangeable). Before cracking the box and starting down the buy parts rabbit hole of rebuild let me check the parts. Again, it’s only from memory, I can check. Otherwise maybe PureMr2 also has a blue ring to compare size to. If these are different then might be another issue you have.

I checked the mechanical and electronic speedos for consistency in length / speedo gear width and teeth - both being the same only that the electronic speedo gear shaft is longer (longer as in a sitting inside the speedo sender, however depth of the speedo gear inside the box is the same on both). So both mechanical and electronic speedo senders (gears) should not cause any compatibility issues regardless of the inside ring colour.

I couldn’t find the blue ring to compare to the white however from memory different mounting as in can’t be swapped (couldn’t use my old non lsd early model speedo ring with lsd diff - had to buy the white).

Process of elimination, if you have tried, swapping the cluster, swapping the electronic speedo drive unit, tried spinning your speedo drive gear with a drill and output is consistent at a static speed, that would support the look into the gearbox.

But if guidance. Gearbox rebuild parts ordered last year, basics: seals, bearings, square bushing, drain bolts, selector boot (basics to rebuild / refresh, not including any synchros etc) with postage was $300+