@laaudionut, I don't have the broad experience like some (lewm) but hopefully can offer some helpful information.
First, if at all possible I would encourage you to find a way to keep your 301 long enough for a direct comparison with whatever replacement you find. I've never had a 301/401/etc. but they are the top choice for many. Others prefer a quality DD, so you need to determine which you like.
Second, there is great potential with some of the Japanese DDs such as you've mentioned. But because of age any of those need to be "refreshed" (parts and calibration) by a knowledgeable technician if that was not done recently. And sometimes parts can be difficult as with the Denon and SP-10 Mk 2(A). Some of those motor units may not come with a proper plinth or a tone arm. So depending on what you find used the costs can quickly add up.
Third, given the potential cost for an upgraded vintage Japanese DD, are there any new options that might be simpler and cost less? Technics has introduced several upgraded models over the past few years, and there is a new Denon, though I don't know anything about it.
Good luck.