Trust me,
@Mano1110786 and I have the same situation, same concerns and same questions. But I think nobody has an answer with the current situation.
You will need to provide all the documents as the first time.
You can ask for the original RTD back in a clear explanatory letter IF you have a valid visa on it. But if there is no multiple entry valid visa, they won't give it back so no need to ask for it either. Just make photocopy of all pages for your records.
No idea if they are even accepting RTD applications by mail or in person.
Every country asks for at least 6 months' validity of passport/travel document not just while applying for visa but also while travelling and returning back. So, no matter where you want to go and come back to Canada, you need to have at least 6 months' validity upon your return date. I am in the same situation as yours.
However, I do not see improvements in travel restrictions and the pandemic anytime soon. So honestly, wouldn't advice anyone travelling even if they do have valid document and visa. But at the same time, I think, wish and pray every day to travel and meet with my family. So I understand completely your situation.