OP—The milling of your slide will remove weight almost proportional to the weight of the red dot sight if you get a small one, like the Shield RMSc. Reliability of slide function probably won't be an issue, but adjusting recoil springs could overcome that.
Using other gimmicks to mount an RDS are second rate solutions that place the RDS too high above the bore and almost always eliminate the mounting of co-witnessed back up iron sights.
The window of a milled RDS serves as an aid to close range (point) shooting inside of 5 yards. If you can see your target in the window, fire.
However, when you learn to use a red dot properly, if you present your gun at eye level where you could see the iron sights, you will see the dot. Learning good consistent presentation is the key. If you have to find and align your irons to take a shot, the red dot will be right there anyway, at whatever range.
If you have good presentation so the dot is always visible in the window, you will be able to shoot fast and accurately no matter the range. Because the dot will place the bullet on target under the dot at any range inside of 20 yards, while you focus on the target and NOT the irons, and the dot can be anywhere inside the window, you have much greater latitude and speed than aligning the target and front and rear irons.
Irons are required on a fighting pistol. By aligning co-witnessed irons you will train yourself to see the dot and learn consistent presentation. After that you will ignore the irons, except if the red dot fails. Then the BACK UP iron sights are useful again, as long as they co-witness with the dot at your chosen range, usually around 10 yards.
Good luck with your project. It's going to cost some coin, but red dots are the sights of now. Notches and posts are the sights of a hundred years ago, no matter how big or colorful. The idea that the human mind and eyes can focus under life-and-death stress on three different planes and shoot well and fast has been disproved. Optics.