Well, one thing to mention is the placement of the last comma. There is much debate as to whether the translation is actually “Truly I say to you, today you shall be with Me in Paradise” or “Truly I say to you today, you shall be with Me in Paradise”, which has slightly different meanings in regards to ‘today’. The first one meaning that on that day, he would be in paradise, and the second meaning that Jesus said that on that day.
Also, what you have to remember about purgatory is that it is a purification process for us to reach heaven. This verse says nothing about the thief immediately going to heaven after death, so he could have made a pit stop in purgatory before he made it to heaven. Also, even though we have an indication of some kind of “time” when going through purgatory, there is no indication that it is in line with time on earth, just as time in heaven isn’t necessarily the same as here on earth.
There is also debate about what is meant by paradise, whether that is heaven, or whether it refers to Abraham’s Bosom, as that is where Jesus when after his own death to preach to those who resided there and bring them to heaven.
Just some thoughts.