I fly on WestJet frequently - usually once or twice a month for work. You do not need to pre-select your seat. It's a cash grab. most airlines are doing it now. 24h before your flight, you check in on the app and can select your seats for free at that time. Provided everyone in your party is booked on the same reservation, they will already have you seated together somewhere. You can get into the app and change your seats, or just live with what you have.
Really, it makes no difference where you sit. They all get you there at the same time. I have a strong preference for window seat, exit row and have yet to not be able to get that on any of the flights I've taken. Just set an alarm for 10mins before your 24h check-in mark and do your online/in app check-in on time.
The ONLY seats that are "better" are the premium seats, and they're only better because someone isn't immediately beside you. There is no first class. There is no business class. Just get the Econo fare, it's wasteful to get anything else.