Disclaimer: haven't flown to the US since a couple years before COVID hit, but here's my two cents:

a) YVR is a US pre-clearance facility, so you should arrive early enough to account for having to pass through customs. Generally, the recommended time is 2 hours prior to your flight, but I often do 1.5 hours without much issue, even with a checked bag.

b) WestJet is not very strict at all with carry-on bags, but of course, YMMV. As long as you are within a couple inches of the dimensions on their website, it won't be an issue with 99.999999% certainty... especially if you're in premium. If your flight is full, they will often request volunteers to gate-check bags free of charge, if you so desire.

c) TSA pre-check is not recognized in Canada, so no... but Premium gets you into the priority security lane (follow the marked NEXUS lane), so that is essentially equivalent. Sometimes the person scanning your pass at security will not be aware, so be sure to ask them if they direct you into the regular lane.

d) The only issue with having a dedicated pre-clearance terminal for US departures, is that lounges located in the true "international" terminal are not accessible, and neither are the domestic lounges. AFAIK, there are only two lounges in the US terminal, a closed Plaza Premium lounge you can access via top-tier AMEX cards (that has no estimated date of re-opening, and is shite anyways...) and an AC Maple Leaf Lounge.

e) YVR is a nice airport, and I could see people maybe wanting to connect through YVR if going to Asia/elsewhere in Canada, given its relatively low passenger loads, ease of exploring the surrounding area for long layovers, etc... but WS really doesn't offer anything special over any other mainstream carrier, IMO.

You'll still have a relatively okay experience, but their lack of lounge access for customers, even paying premium fares, is lame. Their on-board product is slightly lacking behind their major competitors like AC... they don't have seat-back IFE (app-based, tablet provided in Premium). The only saving grace, is they finally made Premium a proper 2-2 layout within the last few years, rather than a 3-3 with middle seat blocked. Haven't tried the food in PY on a 2.5+ hr flight before, so cannot comment on that.