Yeah, my general philosophy on phones these days is to use the OEM rom until either it gets slow and rubbish, or it stops getting updates, then switch to LineageOS or something. OnePlus has done a pretty decent job of not making my phone shitter every time it gets an update, so the OEM rom has lasted much longer than I expected. I reckon with a custom rom it'll last me another 2-3 years at least, which is great value for a phone I bought 3 years ago for £290.