what will happen to my PPT (365 & 2016)

That's correct some features are only available with an Office 365 subscription, same for features like https://support.office.com/en-us/article/Research-for-your-paper-easily-within-Word-1728f286-8702-4d72-8169-ab7677ca0e1f in Word and https://products.office.com/en-us/office365-exclusive.

Only having the subscriber version for a month say, will only gain you access to those exclusive features for that amount of time.  Going back to the single buy version won't help, as that version won't have any understanding of the exclusive subscriber features.  Also, as you may know already, the versions https://support.office.com/en-gb/article/Office-installed-with-Click-to-Run-and-Windows-Installer-on-same-computer-isn-t-supported-30775ef4-fa77-4f47-98fb-c5826a6926cd, so you can only have one at any time.

I think it might just be a case of working out whether it's worth paying for a subscription and all the benefits it brings or just sticking with what you have got and doing out with those other features.