if my phone is rooted and i have twrp, can i update without wiping the phone 1st?
assuming you didn't flash other room, yes. but go to dev menu and make sure the option for cm to update recovery isn't checked or you lose twrp. you might want to update twrp before hand if it's really old.
Should i get a one plus or wait for a new model anouncement? need a new phone really bad.
new one isn't coming until Q3 or so. if you need it now the phone can easily last a couple more years imo