I used to have a Nikon d3300 (bought on Craigslist with 2 kit lenses, plus bag, and accessories for $300) for 6-7 years. Then I wanted to do nicer portraits so I got the 35mm f1.8 for like $200. Then switched to the Sony system 3 years ago because I wanted to do more wildlife which would've required a new camera body because d3300, while an awesome camera, was not it for autofocus and fast shooting. Because I'd need to invest quiet a bit into whatever I chose next anyway, I decided to go with an a7iv, and have been really happy with it. It's perfect for all the stuff I was doing with my Nikon, along with the wildlife capabilities I was looking for, and for my needs I probably won't need another camera body (unless this one stops working).

I'd recommend if all you're looking to do right now is macro, your current body is likely sufficient and you'll probably find nice used macro lenses for it on a budget (<$500). I'm don't have experience with canon so I can't recommend anything specific, but I'm sure Google can help with that part. If you do decide that you are ever hitting the limits of your current body or are thinking about more expensive professional lenses (think >$1200), that is when I'd consider switching up a mirrorless. Either way, I definitely don't think you should invest in a new DSLR body right now.

Also - if you're looking at new mirrorless, the canon system right now is kind of expensive when it comes to nice lenses. I think Sony is the cheapest because of a lot of 3rd party options. But the price for the bodies is more or less the same. Then there are the m43 brands which I don't know much about but might also be a nice option.