Regarding shortening/re-positioning the transfer port, two interesting designs you might enjoy looking at are the famous FWB 300S match rifle, and the little-known Haenel 312 target rifle made in the former East Germany.
On the FWB, the breech is a cone shape which “reaches into” the breech seal, so to speak. This has the effect of shortening the transfer port’s length to almost zero; the head of the piston comes within 1/8” of the pellet when the gun is fired. This is a key ingredient to the recoilless action of these guns, as it greatly reduces the necessary spring strength, and thus how much recoil must be overcome. These guns shoot well over 600 FPS and you can literally cock one with your little finger, which is quite amazing.
The Haenel has frankly the weirdest transfer port and breech seal design I have ever seen. It has a fixed barrel with sidelever cocking and sliding breech sleeve similar to the FWB, but the transfer port is a long “finger” which also serves to push the pellet into the barrel as you describe. The gun has no resilient breech seal material, but reduces leakage at the breech by creating a such a long and angular path for the air to escape there, if that makes sense.