what I did with both my HK45 and HK45C was purchase a stripped slide for machining. I then bought the "guts" for the slide, and assembled them (minus sights). I then sent the "extra" slides to Ashbury Precision Ordnance (APO) https://www.ashburyprecisionordnance.net/ , had the slides milled and had Heinie suppressor height irons mounted. Now I maintain by relatively low numbered German original slides (I've had both since the first US release of the platform, so it was important to me to keep the original slides OEM), and slides that have RMR-2's on them. I have been exceptionally pleased with the work and CS at APO.
OK, all of that said, I don't see the P30SK on their list of HK pistols they install red dots on. However, I know you can call and talk to them, I just don't know if they are putting red dots on the SK size slides, as space is limited. I suspect you'd not have room for a rear iron sight (that is an assumption on my part)?
And, after babbling on about APO, I note you already have a machinist lined up, I am just relaying my experience; go with which ever vendor/machine shop you have confidence in. Keep in mind, this is not a cheap or inexpensive endeavor you are setting out on.
That said, it took me a while to warm up to them, but pistol mounted red dots is where it is at, once put the time and training into using the red dot. I mean a lot of time and training. Check out this Surefire Field Notes video about pistol mounted red dots.
SureFire Field Notes Ep. 49: Red Dots on Handguns with Scott Jedlinski - Soldier Systems Daily
good luck, I hope my rambling reply is somewhat helpful.