To me, pleasure has been defined in PD18 and a variety of previous PDs beginning with PD03. I think PD19 is referencing those, and pivoting to the subject of craving infinite time for infinite pleasure. PD20 then mentions "fears about a life after death" and "departure from life", which seems to indicate that this is the subject of these two PDs, not pleasure. The two are describing the temporal limits of a person and how to think about it. [Parentheses are my additions.]
PD19. Infinite time contains no greater pleasure [the body’s goal] than does finite time, if one determines the limits of pleasure rationally.
PD20. The body takes the limits of pleasure to be infinite, and infinite time would provide such pleasure. But the mind has provided us with the complete life by a rational examination of the body’s goal and limitations and by dispelling our fears about a life after death [as described in previous PDs] ; and so we no longer need unlimited time. On the other hand, it does not avoid pleasure [which is the body’s goal, after all], nor, when conditions occasion our departure from life, does it come to the end in a manner that would suggest that it had fallen short in any way of the best possible existence.
Imagine that you are experiencing a bliss that's so great that you want it to last forever. At some point you'll bring yourself pain if you start to crave the experience. These are saying that you need to understand that your life is finite, so enjoy your bliss. Don't crave it, but don't reject bliss because it's going to end: relish it for what it is, and relish your finite life for what it is.
So I think that if you try to understand PD19 as saying something about pleasure specifically it becomes confusing and mysterious. At least for me, it becomes much clearer and simpler as I'm trying to describe it. "Now that you have a reasoned understanding of pleasure and of the material universe, think about what it means to have a finite life in an infinite universe." This, by itself, is a lot to think about.