I want to point out that even if diseases get introduced earlier the Natives still would suffer somewhat later when European contact gets stronger because they obviously wouldn´t get the full package and later Eurasian diseases.
It all depends on how contact and European presence develop. If we accept the idea that disease was by far the biggest killer of the contact era (which I do believe), and actual conquest etc. only played a secondary role when it comes to the number of native Americans' dying, then any European presence is going to lead to a huge "dying off" anyway. So, yeah... if we're dealing with temporary contact around 500 or so (only to be re-established around OTL's moment of contact), that's actually
worse for the native Americans. They get hit with terrible disease around 500, take a huge hit from that, and then go back to isolation. No resistance is built up to constantly adapting disease in Europa, so a thousand years later... it all happens again. Net effect: it's one more vast wave of epidemics around 500 that they have to recover from, which only weakens their later developmental position when it's time for "round two", while giving them no benefits at all.
On the other hand, if contact is sustained... then it's easy to see the vast epidemics around 500 being terrible, and a wave of disease gradually burning its way trough the "New World"... but after that's over and done with, the lasting contact eventually
does result in a resistant population. That requires pretty intense contact, though. And the risk of that scenario - for the native peoples - is European powers starting to colonise as in OTL, just way earlier. Of course, back in 500 large-scale colonisation is less likely...
A situation where some European power establishes lasting trade posts on the eastern seaboard, but not much beyond that, is not terribly unlikely...
once you've gotten past the POD and actually established reasons and means to
get to America. Initial contact then leads to disease, if the trade is profitable it leads to more trade posts, which leads to European diseases basically burning trough the Americas in a number of waves... and after that, resistance to those European diseases becomes a fact. Over the next hundreds of years, the native population numbers gradually recover, and maybe trade with European powers (coupled with a lack of mass settling) could result in greater technological parity by the time European powers get to the point where mass settlement becomes realistic.
It's perhaps not the most likely outcome, but it is a possibility.