Seems like you are pretty focused on Canon.
But before actually making the decision of investing more in either glass or body, I'd say, decide on a system you'd want to pursue. Looking at your current setup, I don't think you are that heavily invested into Canon to say that a decision must be made with a Canon product.
Personally, I think DLSR days are numbered. I shot with a Nikon D750 semi-professionally. Although it got the job done for the most part, it does have some problems.
1. If I had trouble using the auto-focus, most likely due to low light. Then the manual focus system is almost un-usable to me. I know there is the focus indicator, but if auto-focus didn't work, the focus indicator won't really help that much either. The keep rate in these occasions are really low for me. IMO, focus peaking should be a basic feature in modern digital cameras. The Nikon D850 has them, but it's quite an expensive body.
2. The limited AF points that are only available around the center of the frame. I understand you can focus and recompose, which is almost a must for these DLSR bodies, but it's very hard to nail focus when shooting on wider apertures.
I was in the same camp to say "always invest in your glass first." However, that's no longer true. The newer high pixel count sensors, specifically, the 40MP+ sensors on the Nikon D850 and Sony A7Rii and A7Riii really exposed some of the short comings of some of the older lenses. I believe there is a review floating around the internet somewhere detailing about how some of the Nikon lenses felt obsolete. So if you decide to invest in more glass, keep this in mind. There is no silver bullet to say any lens design would always stand through the test of time.
With the advancement of digital technology, there are so many things that modern body can do. Some of the features I love and look for are things like:
1. Stablization, whether it's in-body stablization or the availability of lenses with built-in stablization. Better yet, hybrid stablization, look at Olympus, Panasonic, and Fuji for example.
2. Edge to edge AF points. This just allows for way more freedom in composition.
3. Focus peaking and live view magnification. This is for manual focus, focus peaking is especially nice because it allows for fast manual focus when you are shooting objects that might be moving.
4. Eye-AF from Sony. The Eye-AF in the third generation Sony cameras are simply amazing. It just makes any portraiture work much more enjoyable.
5. Fast AF, cameras can always do better in this regard. Even the D750 have some predictive algorithm when it tries to focus on moving objects, doesn't really work that well all the time, but better than nothing.
My suggestion would actually be to look at what kind of technology is available, what you think would be essential, then make a decision on whether you want to stay with Canon or not. The T3i is a pretty old body now that almost any upgrade to it would be a huge upgrade in every way.