I prefer the original F screen. This is not because of brightness but because it has a plain matte surface, without the distraction of a micro prism spot. I have had a FT, now sold, that I had the meter removed and a full surface mirror installed instead of the half surface mirror necessary for the ttl meter. There was some improvement but nothing dramatic. I also have the 20mm f3.5 Zuiko and I focus by scale for distances beyond 5 or 6 feet. I generally find that focusing closer than 6 feet, using the screen, acceptable down to interior room lighting. (Illumination that would give you 1/30 @ f3.5 at ISO 100)
If the light is dimmer than that then if I can find a specular reflection off some surface I can usually focus on that.
Remember, you focus on contrast, not just brightness.
Here is an idea to try with your FT and the 20mm lens. Try focusing using the matte portion of the screen, just above or below the micro prism spot, or to the left or right if holding the camera in landscape orientation. This may help, depending on your eyesight. I'm 67 years old and my eyesight is certainly not as good as at 22, my age when I first bought a couple of Pen F cameras.