Safariland has a Grip Lock System line of holsters: https://www.safariland.com/category-landing/gls-holsters.html
I looked at serpa holsters but they didn't have one that I could find for the Ruger SR45. Stumbled across the Safariland and picked that up instead. In short, your index finger trips the release when you wrap your hand around the grip to draw.
I made the same choice for my SR45 when I found out that there isn't a Serpa that fits it. My current EDC is an SR9, but I foresee rotating the SR45 in at some time. To maintain the same holster style between the two I had to go the adjustable Safariland route. Just an observation, but if you are releasing the Grip Lock with your trigger finger you are doing it wrong. The middle finger, pressing into the bottom of the trigger guard, is what unlocks the pistol. Your index finger should be alongside the holster, dropping into place alongside the slide as you draw.