Forget the expiration date of your PR card, just ignore it (for now).
These are the questions you need to answer before a determination on your compliance with the Residency Obligation (RO) can be made:
What day did you become a PR?
Since that day, how many days have you been in Canada?
What day will you be coming back into Canada?
Now, using the answers to the questions above you can determine whether or not you meet (/can meet) the RO.
The laws regarding the RO state that on the date of examination (i.e. crossing the border, applying for any document from IRCC, etc) you must have been physically present in Canada for at least 730 days in the 5 years immediately preceding that day.
However;
Presumably, in your case, you have not yet been a PR for 5 years. In that case, you count the days you've been in Canada since you became a PR, and add to that the number of days still left until the 5-year anniversary of you becoming a PR from the day that you return to Canada. That number must be 730 or more, or you are in breach of the RO and you risk being reported on entry, or examination.
If you are reported for a breach of the RO, the process will start to revoke your status and remove you from Canada.
Where the expiration date of your PR card becomes an issue is if you wish to travel to Canada by a commercial carrier (plane, bus, train). In most cases, you cannot board a commercial vehicle with an expired PR Card as a PR unless you have a Permanent Resident Travel Document (PRTD). From what you describe, this shouldn't be an issue if your PR Card is valid until 2021 and you wish to return before then. However, that's the only relevance to the expiration date on the PR Card. Like a passport, it is a TRAVEL DOCUMENT and not your status.