If you qualify for a PGWP, that is the better option for you. You can work part-time right now until you get written confirmation of program completion. Once you have the necessary docs, you can apply for a PGWP and can then work full-time right away. If you get written confirmation and don't apply for a PGWP, you must stop work that day.
Applying for the inland sponsorship OWP won't allow you to continue working after your receive confirmation of program completion. You would have to wait for the OWP to be approved before you could work again. Given the timeline, you are likely to become a PR first (depending on pandemic delays, of course).
If you don't apply for either work permit and haven't become a PR before June 30th, you should apply to extend your stay as a visitor in order to maintain status while you wait.