It’s really good for stuff like this to go around every once in a while! Strange as it may seem, people may in fact migrate here from Twitter or Instagram, where this stuff works differently and where there are different expectations of engagement.
DON’T FORGET - *most* Tumblr users DO NOT MIND if you engage with their OLD posts! (Apparently on Instagram they do? this baffles me.)
Many also don’t mind if you “spam” their notifications with a bunch of likes or reblogs in a row.
Tumblr has a rich culture of Very Old Posts continuing to make the reblog rounds, and people become fond of them.
Also, unlike Twitter, you can reblog the same post multiple times. Heck, you can reblog the same post every hour on the hour for days. (Please don’t.) But you do see a lot of “oh this came across my dash again, must reblog” with posts users are fond of. This is fine.
Tags ARE a divine mystery. People use the tags both for organization (inasmuch as this works, sometimes), and for added commentary. Commentary added to the tags will generally be seen by those who follow that person and see their reblog on their dash; but the OP and whoever they reblogged it from can also see the tags in the notifications.
So again – you can use the tags for commentary, and many people do. But people WILL see it. It just won’t “stick” with the post… necessarily. Tumblr also has a culture of people seeing some tags they think are relevant or clever, and reblogging a post with someone else’s tags included. So bear that in mind as well – something you put in the tags could get “pulled up” into a reblog chain by someone else, and this is generally seen as fine.