I always feel like around the 2 year mark my phone feels very different compared to when I bought it. Battery feels worse, everything feels slower, and crashes start to accumulate. Are you not running into this?
I have experienced that as well, but I feel it can usually be remedied by doing a factory reset. I also typically never do in-place upgrades for Android updates. I will download the full OS images (or the RUU/whatever updater is required for that brand phone) and do a full reset and restore. I will say for some reason this seems to be less of a problem on Sony Xperias that I have owned versus LG, HTC, Motorola, and others. But I for sure know what you're talking about.