Outside threshold workouts effective (not intervals)

Starting last October I did TR LV SSB and SPB. SInce it has warmed up, I have been riding outside. My goal is to improve my time up the biggest local hill which is also a Strava segment called “Rails to Resort”. It is (for me) a little under 50min effort (PR is 48:57) A lot of my rides are up this same hill at SS intensity

My question is whether this is an effective training technique to continue to improve my times, or whether I need to do some work at a higher intensity (above FTP)? Is riding at SS and Threshold enough to keep pushing my FTP up?

I’m 58yr male currently about 3.5W/Kg