Just wanted to post a follow up here, for posterity:

-Purchased the car, drained and filled w/ Pennzoil Syncromesh. Old fluid drained with odd sturdy air bubbles. No change, possibly worse.

-Drove 50-ish miles, drained and filled with cheap synthetic ATF. Fluid drained with no air bubbles. No change in driveability, but wasn't as noisy.

-Drone 100-ish miles, drained and filled with Redline MTL. No bubbles. This time, there is a slight improvement in the 3rd gear issue. Before, I had to wait a full 2 second in neutral and let the engine spool down completely before selectring 3rd. Now, with the redline, I still need to pause but only for about a second.

In addition to the above fluid changes I have done a bunch of other things like look at linkages, change engine and trans mounts, tighten the hex bolts etc. the conclusion is my synchro is worn, but Redline and careful driving make it liveable for now.

Side note: the general opinion out there seems to be that these 4-speeds are bullet proof. However, I've met 3 different people recently that said they have had the same issue as me. My 350k mile automatic transmisson on my 300D, however, still shifts fine. This is andectotal, of course.

(big thanks to Shern for the Redine recommendation. It made this car much better to drive)