Very wise choice and potentially without some of the issues that are somewhat prevalent with the new 11 or 12.9. I bought an 11 LTE and returned it since out of bend "spec", deciding instead to continue hot spotting with my phone and my 10.5. 10.5 is more than capable for years to come unless you are a 2% power user which I'll let someone else define.
Yes, there are some 10.5s with a bright white spot on screen a little above the button, but mine has not developed this in a little over a year. I usually run 50-60% brightness though.
I bought one a couple of months ago and this thing is so blazing fast, I can't imagine that it won't be fine for 5 years. I'm using a 6s that was released over 3 years ago and is still almost as fast as the newer phones on IOS 12. I don't think you have anything to worry about unless Apple suddenly drastically redesigns IOS and if the 10.5 suddenly becomes unusable then so will millions of other devices out there. You'll be fine with a 10.5.
If you look the chart on Wiki you can kinda get an idea. One of the iPad spans over 5yrs of updating. I'm guessing that it stopped was because it was 32bit? Current iPads and iPhones are very similar (64bit, touch i.d. or face i.d. camera). Maybe this will be 5 or 6yrs. You can still use it after that. Some of the Apps are supported. The upside is that there is more space to work with vs the earlier models.