While I'm not a novice, I am also no professional. I do appreciate your helpfulness so I hope my response doesn't come off as snarky.
I have the kit 18-135 and Canon's 100-300mm. I'm a working man on a budget which has kept me from getting a faster fixed length lens for low light. But first/next accessory I would spring for is an add on flash. When I shoot weddings or the like (as favors or for my own personal collections) the on board just doesn't cut it.
I shoot kind of everything. My preferred subjects have changed over the years as I have added kids (7). I'll shoot architecture, moon & stars, sunsets, birds and wildlife, sports, concerts, portraits and family pics, etc etc etc.
What really brought up the question was some shots I did on a recent vacation. One day was spent at a botanical gardens where I was doing macros of flowers/bees/butterflies and broad shots of landcapes/overall scenery. I brought my 100-300 for that but never got it out. I also shot a few pics with my phone for quick posts to Facebook. While working in Lightroom on my raw files, I had to reduce highlighting LOT to bring out individual clouds from all gray while my phone shots needed no such adjustments. Similarly, the couple phone shots of bumblebees on flowers were nothing short of impressive while my raws from the Rebel needed a lot of post processing to become passable.
On the Rebel I was mostly shooting AV with iso 100 or 200 and f3.5-4.5 depending on zoom for the flower/bees close up, then would change to f7.1-11 for broader scenery shots.
I print very little. My PREFERRED viewing is on our 65" 4K TV, but honestly it's more on my phone or 17" computer monitor.
Attached are some examples.
Hi again!
Given your subject range and the type of output: frankly, I think you could continue to use the cellphone as long as you don't intend to produce large prints, which you say is not your preferred output. That would also be true for video output. Cellphones benefit from the massive improvements in computational photography that apply all sorts of digital corrections transparently for you. DSLRs are designed to allow you to process according to your own preferences after shooting - although I try to get it right in-camera as I shoot.
Where things differ is for events like the weddings. If you are shooting these as favours to your friends, then you are taking on a responsibility to get good-quality images that can be printed - remembering, this may well be one of the most important days of their lives. For those, and similar occasions, a decent DSLR or MILC (Mirrorless Interchangeable Lens Camera) really is the way to go. You get more control and better sensor size, but you have to take control of the camera and lens much more.
Where a DSLR really shines is, as Ricky alluded to, for its ability to change with different lenses and also its much larger sensor. Now, you could definitely get better results with a newer camera. Like cellphones, camera tech has come a long way since that camera was released in 2011. That said, I have one of the T3i units and got some good results with it - see below.
Where you are definitely challenged is the 100-300mm lens you are using, if it is the Canon EF 100-300mm f/4.5-5.6 USM model, which is all I can find for this range. This is an ancient lens, with no image stabilization. You would get massively better results with the Canon EF 70-300 MkI or MkII lenses that have great optics and stabilization. They are available via the Canon Refurb site, at this link if you are at all interested. For a review of the 70-300 series of Canon lenses see that HERE . I recommend you check that out.
If you were going to get a different camera, then much would depend on the budget. Considering you should upgrade the telephoto lens in any case, you could transition across to the EOS R mirrorless system. There are two offerings, the R7 and R10. The R7 is clearly the superior camera with in-body stabilization, eye tracking etc. - I recommend you check out the reviews for this on DPREVIEW.COM for example. It also does 4k video. The camera is available with a couple of kit lenses of which I would suggest the 18-150mm for general photography and that could do a lot of your normal work. For longer work, such as wildlife, you could eventually get the RF100-400 lens that is excellent and not a massively expensive lens.
If you decide to stick with a DSLR, then you can get new, refurbished or second-hand the EOS 80D or 90D, and either will be far superior to the performance of your T3i. You should still replace the 100-300 lens as that is, IMHO, the worst weakest link.
