My guess is that more volume (i.e. time per week) would result in more FTP gainz than lifting or gaining weight. Lifting would probably make you a more well rounded athlete though. You have had good increases over the last couple of years, when that slows down you’ll need to up the training stimulus, either with more intensity or more volume or both, but eventually it will top out.
I think the real problem is that you need to move to an area that is more attuned to your body type. In that type of terrain everyone is probably talking about improving their w/kg and getting down into the 140s and you are already there!
My perspective is that it is a matter of perspective. The physically smaller but high w/kg riders tend to want to get bigger and put out more raw power. The physically larger tend to have higher raw power but want to get smaller to improve w/kg. The grass is always greener on the other side of the fence.
Personally, I’d take that extra 1-2 hours you’d spend lifting, add it to your current 5-6 hours /week and see what gains you get from that. If you can bump to 10+ hrs / week that would be even better. 2 years ago I had a huge bump in fitness because I switched from riding outside almost all of the time to riding indoors during the week and outside on the weekend. It is more time efficient for me and I could get the same or slightly more saddle time in and better quality (no wind, traffic, traffic lights, etc.).