And since I bought a used OnePlus 3 in late 2017, when should I start looking for a new phone?
Still using my 3T. As for when you should start looking for a new one: it's entirely, wholly up to you. I am planning to stick with mine for 2-3 years more at least, since I would gain absolutely nothing useful from buying a new one. I don't buy phones based on looks and I don't care about trends; I buy my devices based on needs and the 3T already fulfills my needs.