A larger fuselage isn't necessarily easy to see...there are a lot of factors involved. The Splendor is a big airplane and sometimes was invisible in the air. I'm not convinced that a gigantic fuselage is the answer. Seba's airplanes have been on the smaller side and it hasn't affected him at all. I think a well-balanced fuselage is the answer.
I think ink pertaining to this issue, having a "tight" airplane is key, so making adjustments as you've done to the strut mounts I see going a long way. As with most things related to F3A, I am under the impression that it's not one big thing, rather several small ones. I'm sure pilot technique is applicable to a degree as well. I'm very close friends with Quique Somenzini and was his caller for Argentina and Portugal. I've flown with him a bunch and built/wrenched on airplanes for him. I can tell you that he's quite particular with the build and setup of his F3A models and does a lot of things that seem ritualistic, but clearly have some merit as his airplanes were always settled in the air. I don't have the luck to fly with Andrew Jesky on a regular basis, but know that he tweaks and tunes models a lot. CPLR is a mystery to me, but is clearly quite skilled in this department as well.
I wish you all the luck in the world. I hope that as you try some things out, you report back here with your findings! Curious minds want to know.