1. Quit cleaning your barrel between groups. Get it dirty and keep it dirty while working on load development. If you live in a dry state with low to no humidity I'd quit cleaning it all together until the accuracy starts to go bad. Some of my LR rifles haven't been cleaned in over 1300 rounds. They still shoot 1/4 moa 5-shot groups.
2. Look for seating depth differences in .020" increments; .005" is for fine tuning.
3. Once you find a sweet spot for seating depth, then you'll want to play with powder charges; more or less. I'd start by running a combined pressure/accuracy ladder at 300, 400, 500 or even 600 yds if possible. Take a 100 rd MTM box and prep all 100 cases. Then, charge all the cases. If 43 is book max, the 1% of 43 is approx .43 gr. You'll end up with 10 sets of 10 cases. The max powder charge should be 44 gr. I'm not familiar with the 6.5x55; 44 gr may not even be close to your rifles max in a modern rifle such as yours. Charge 10 cases with 44 gr and set them in the top row of the MTM box. Then, 10 cases of 43.6 gr and put them in the 9th row. Then 10 cases charged with 43.2 gr and set them in the 8th row; and so on and so on downward until the 1st row is 40.4 gr. You end up lowering the powder charge .4 gr for each 10 cases. These are now, all set in your 100 rd MTM box with NO bullets seated. Obtain a light loading press such as the Lee Jr press which I hand hold while seating bullets at the range. Set up your die, pre-adjusted to seat the bullets at your desired seating depth. Go to the range. Have 1/2 dozen additional loads already seated so you can use them to get your zero at the range you choose to shoot the ladder.
Now, for the target. I like a 4'x4' sheet of plywood. I cover it with a 4'x4' sheet of black plastic. I then take a $.97 cent can of flat white spray paint and spray the whole sheet of plastic until it's white. Take a can of orange spray paint and paint aiming points. Place the target at the farthest distance you can. I've done my last few loads @ 600 yds. I Use the 1/2 dozen loads to make sure I can hit the target with my ladder loads. I then take the first 40.4 gr load and shoot it at the aiming spot in the middle of the target. The dried white paint will flake away and with a decent spotter you should be able to clearly see the hit. You basically have an extremely large Dirty Birt/Shoot N See target. Mark it on a clean sheet of paper their at your bench. I shoot prone and don't even worry about velocity yet via my chronograph. Now, take the 2nd charge increase, 40.8 gr and shoot it; repeat and rinse until all 10 charge increases have been fired. You should have something like this.
As near a windless day as possible is best, or at least a constant wind from one direction as possible. You see 3 shots increasing in height, then 3 shots at the same level (looks like 2), then two more shots increasing in height again. The 3 shots are the accuracy node. I then take those 3 powder charges that are in the accuracy node and load up 3 of each. Shoot them at the other orange aiming spots. I then take the best groups and load up 3 more of the same powder charge but with a different seating depth; in and out .01" or .02" and shoot them. My MTM box looked like this when I was done.
I had a .2 to .3 moa load, all within 32 rounds fired. Should have been less because I wanted to explore a higher powder charge. The beauty of this is I don't have to pull loaded rounds at the house and I typically have enough pre charged cases to pursue a potential accurate powder charge and play with seating depth a bit. Once I settled on a powder charge, I then went home, dumped out the powder from the other pre-charged cases and recharged them with the accurate powder charges. Back to the range with the chronograph for a solid 100 yd zero and velocity readings.
Your mileage may vary