There is only one round of PL fixtures after the May the 21st (the w/e of the 28th). The EFL season will have finshed on May the 6th with the PO finals to played over the long weekend of May 27th > 29th.
I disagree that POs tix will be easy to get, because the fixtures and clubs are not confirmed until a few days beforehand and they are highly sought after even the Legaue 2 games (I know 1st hand, being an exile of a L2 club who has made the final 5 times) and there will be lots of restrictions and they are sold through club memberships to reward the the loyal fans who have attended home andaway games as well as policing issues - if they go on general sale for neutrals it will be a day or two before the match.
If a well supported club plays a less well supported club in the PO final, there will be restrictions in place to prevent supporters of the well supported club buying tix in for the the less supported club's end.