Although not directly related to BA, I'll be using avios and booking this through BAEC. My choices are YVR-JFK or JFK-YVR with Cathay (I believe this is almost daily) / LAX-JFK or JFK-LAX with AA (3/4 flights a day). Without status I'd like to try any of these First class products across North America. These flights are all 50k avios plus a few dollars.

Bearing in mind the flight times, with some flights being overnight (this will of course save the cost of a night in a hotel, but I can imagine it might be a case of lights out and expected to sleep) and also departure/arrival lounge access, does anyone have any recommendation for the best overall First Class experience? I'm very flexible on my trip and can do any of the routes mentioned above.

Hope this is the right forum to post this, and thanks in advance for any advice.

Although probably not the right forum, it is a no brainer. CX First especially on JFK-YVR. Get to use the new AA flagship dining at JFK and it is westbound so don't feel like you have to sleep.

In terms of absolute product, CX F is the way to go. The AA F Transcon product is very good, but CX is better.

AA transcon seats are very similar to the CX business class seats. The First class on CX is significantly better.
Cannot say anything about the CX service on this specific flight, but AA in F is pretty mediocre in my opinion.

Be careful here. Eastbound "overnight" services can be relatively short flights, i.e. under 5 hours from takeoff to landing, which means < 4.5 hours reclined in one's seat. ime, this service does not "save the cost of a night in a hotel" unless one has rights to extra, extra early check-in for the next night (or are returning home). Plus, arriving at JFK at 4:45 am (the actual gate arrival time of AA's flight 10 most days last week), could be extremely problematic.

The westbound CX flight is my recommendation, especially if you can land an affordable, discount rate at the Fairmont located within the airport terminal (YVR).

+1. Eastbound is too short to enjoy CX F to the max.

Actually, IIRC, they are the same seats from the same manufacturer.

Great advice from everyone here. In JFK what are my lounge options as non-status card holder? Are there any arrivals facilities I can take advantage in YVR?

On a couple of short hop BA Sixth Freedom flights in the Caribbean, the planes are just given a quick once over then up and away. Do these fights offer the full First Class CX experience or are they seen as the black sheep of the family?

Your F ticket will let you access the AA Flagship lounge at JFK. This lounge has recently been totally revamped and is quite nice.
No arrivals facilities in YVR that come with your ticket unfortunately.
And these flights do offer the full F experience, adjusted for the rather short flight time. Remember, the flight originates/continues on to HKG (YVR is merely a short stop) and service is designed for the longhaul passengers. Check SFO777s trip report for a detailed review, for example.

But...but...but...there's no caviar!!!

The JFK-YVR (and YVR-JFK for that matter) does not have a 'full' meal service as in that of long haul flights out of HKG, but there is still a bit of quality food of course (especially out of JFK where CX uses DO&CO). But OP can still get goodies like PJs and amenity kits. (Not sure about Lynch Bages?) There is a risk also, that YVR based crews may be working OP's flight, who in some anecdotes are not that different from AAL crew.

Of course these are relatively minor issues and for sure the onboard product and experience is a tad (or a lot more) above the AA A321T. But in defense for AA, I love the private jet feel of the 321T and the (occasionally and by-chance-only) lovely AA crew too, plus the ice cream sundae! (Speaking as someone who loved CX so much that I switched from AAdvantage to CX's own program). Also, if OP does LAX-JFK or vv, then they can enjoy Flagship First Dining at both locations (plus a third meal at QF F LAX if desired for ex-LAX to JFK).

The AA TCON F seats are very similar to a standard Cirrus installation but the installation is not identical. The AA seats are a bit closer together, you can see this if you look a the gap at floor level that one might typically swing one's foot through as one enters the seat. It's a lot smaller gap.

It is still a very fine seat, certainly the best inside the USA. Service can also be very good, I took a JFK-LAX flight a year or so ago where the purser serving in F wouldn't have been out of place in CX F; best AA service ever.