Be aware that a load that is too light may not cycle the slide reliably, so without changing the recoil spring there is a limit to how light you can go. Since I have similar issues I've found that I have to limit how much I shoot using lighter pistols. For me it's about 100 rounds with my 40 caliber Sig P239 which is actually a heavyweight compared to your Shield.
One thing that has helped may fall into the category of me fooling myself. But I've recently changed from reloading with Unique it favor of IMR SR 7625, which meters fantastically well in my Lee powder measures. First time out with starting loads with the 7625 I noticed that it felt distinctly soft with a significant reduction in muzzle flip. Stepped the load up to 5.4 grains for a 180 grain bullet and still found it notably soft shooting. However I had my chronograph along for that session and the bullets were clocking 960 fps., which is pretty much a standard 40 caliber range load. In a side by side comparison with Federal Champion my home brew feels softer to me and it seems that muzzle flip is reduced by 40 % or more. To be honest I cannot come up with a solid reason for this, thus my suggestion I may be wearing blinders. However, one possibility is that the 7625 has a slower and more even pressure curve than most handgun powders and is producing good velocity with what I perceive as a softer recoil. BTW, I'm using the same powder in my 45ACP loads and in my Ruger SR1911 it shoots so softly that I would consider it a training round for the recoil shy.
BTW, one additional benefit of the SR 7625 is that it's VERY clean shooting. In fact it's so clean shooting that after 150 rounds downrange with the Ruger I simply wipe the muzzle end of the slide clean with a damp paper towel, clean the front sight with a Q Tip and call it good. Before changing powder and shooting with either Unique or the Federal Champion just 100 rounds downrange would leave the pistol in need of a full field strip and cleaning. I'd also "lose" the front sight about 70-80 rounds downrange due to fouling turning that big white dot brown.
I would suggest that you pick up a poind or two of the SR 7625 and give it a try. With a plated or FMJ bullet a 5.2 grain starting load with a 180 grain bullet is a real and distinct powder puff load.
170gr swc I cast for several years now in my Glock 23. Barrel stays clean and runs the gun just fine. Nice light load.
