Same, running a xiaomi mi 6 since early 2018 with LineageOS and it's still perfect for me. Sure, new phones have better cameras and screen but the difference is not that noticeable. I'm using a google camera app so that improves the pictures significantly so that compensates for my needs
This is exactly what I did on my OP7 (only 4 years old). A new battery, a new USB port and a new back (thank you OnePlus for the back in glass).
I installed another ROM... And the only drawback on that "brand new" phone is that the camera is slow and not as good than the stock one ( even with Gcam or others derivative).