1953 is disputable and 1990 doesn't apply to it at all.
1953 is not legally disputable. It does not matter that Poland was not a democracy at this time, treaties and declarations between dictatorships are just as legally binding. It is called international law, not law between democracies.
And of course the 2+4 treaty and the border treaty and the good neighbourhood treaty all apply since they designed to solve all outstanding issues from WWII, with the 2+4 treaty serving as a peace treaty. The only reason why the latter two were even necessary was because the signatory powers that a reunited Germany needed to reassure Polish fears and give a further guarantee on the border. This would also have been the final opportunity for Warsaw to raise the issue of reparations. Since they didn't, it was considered a closed case and thus the 2+4 treaty concluded it.
As I have pointed out a few times before, Poland's insistence on the border guarantee actually closed all options for them to later raise the issue of reparations again. Because the 2+4 treaty has one possible weakness. It only included the 4 victorious powers, not the other countries that suffered from the Nazis. Thus the other countries could technically argue that they were not sufficiently consulted and their interests not properly respected. A weak objection, but one that could possibly be made. However Poland could not do the same, since Polish objections and wishes were indeed listened to, to the satisfaction of Warsaw which Berlin could easily prove with records and the treaties.
To summarize, the case of reparations has been legally settlef and there is no way Warsaw could legally challenge it. Hence they don't use the legal way, but use it as a moral argument.
A long time ambition of Germany
Let us be real here, that is a totally unrealistic and even somewhat anachronistic goal. Even now it is already somewhat ridiculous, that countries like the UK and France sit on this table, despite only habing a fraction of e.g. China's or the USA's population. Given the predictions in terms of economic and population growth, this disparity will only become larger with time. No doubt India will one day become a member, probably at the expense of the UK. And the French seat might hopefully become a seat for the EU, once we have a truly common security and foreign policy.
being constantly reminded
The current PiS is most likely the last one to raise this issue, simply because time is not on their side. There are still people alive from WWII, and Poland has only been part of the EU for less than a generation, not enough time to profoundly change the outlook of all its' people. This will change however inevitably. PiS will not gove up on this issue (not pressing it either of course) since it would mean losing face as long as they are in government. Once they are voted out - whenever this will happen - the new, probably PO-led government won't raise issue again, and neither will a successor government led by PiS under a new party leadership. Because by then it will have become a truly ridiculous endeavour.