RC_F:
Actually the first thing I want to say is: if you don't think you notice the change in balance much with the tank forward, I recommend (for most people) not changing it and messing with a pump. I really enjoyed flying the plane with tank at CG, but I did not enjoy the issues that come with running a pump with a motor that is not designed to do so. If you are a seasoned glow engine guy and have experience with pumps then this probably doesn't apply to you. But if you are looking to get the most amount of flying in and not worry about tuning and dead sticks, fly electric - uh I mean keep it simple and forget the pump
The one place where you went wrong is
with a forward tank, this location presumably takes into account the effect on the flight CG of some percentage of the fuel in the tank.
The design point for the CG is with an empty tank, or tank on the CG itself. When you fill the tank and fly, your CG is forward of the design point and the plane is nose heavy. If it were the other way around, the plane would be tail heavy when the tank is empty and could be trouble for a lot of pilots.
When I had the plane without a pump, I had the plane balanced so it felt a little nose heavy at takeoff, and a little tail heavy at landing. I don't know if that was the best thing to do, since I think the change from slightly nose heavy to slightly tail heavy could be more noticeable or drastic than always having a nose heavy feeling (probably very subjective though).
There are a few things that could be working on your trim 'issue': it could be the thrust line of the engine (extra down thrust would pull the nose down with power), it could be that the horizontal stab was glued in with a little positive angle (acting like down elevator), or maybe it could have something to do with the speed at which you fly and trim the plane. There are trim charts to work on this stuff, but if the only thing that you are noticing is where the elevator is trimmed when you land, then don't worry too much about it. What matters most is how it flies. I do agree that the amount of shimming you added is a lot.
The 45deg inverted test: if you are adding shims to the rear of the wing, you are making the wing generate more lift when upright. When you roll to inverted, up is now down and the added incidence can cause the plane to pull toward the ground. If it is that noticeable I have a feeling there is something that you are working against (stab angle or thrust line).
Yeah, the Venus is a great flying airplane. I sold mine to buy a Sebart Wind 110 and regretted it ever since. I really liked flying the Venus a lot more than the Wind. I am very fortunate to have come across another Venus and am excited to have it as my backup plane.