How significant 1.2 oz is depends on how much you value weight and how heavy the pistol is to begin with. My two lightest handguns weigh less than 10 oz unloaded so the RMR you mentioned would increase their weight by 12% or more. That being said, the added bulk is more my concern as when I carry a small handgun, it's usually in a pocket holster and when I carry OWB (I've never found IWB to be particularly comfortable) its usually with a large enough gun that extra bulk would be problematic. The extra bulk of the red dot also limits my choice of holsters somewhat and some of my favorite holsters are not available for guns with a red dot.
I'm not so confident in that. The handguns that I shoot out to 50 yards are chambered cartridges that are relatively high-velocity, and thus flat shooting, at least by handgun standards like .357 Magnum, .44 Magnum, and 10mm Auto. With iron sights, I notice a significant difference in POI between shorter and longer distances and the higher sight offset of a red dot could only accentuate this. Is the difference insurmountable? Probably not but I don't want to make things any more complicated than I need to.
I don't think that the difference is insurmountable or that using a red dot will "ruin" your iron sight shooting, but its another complication that I, personally, would rather not add. I own a variety of different pistols with differing styles of controls, actions, triggers, etc. and, because of this, I try to use techniques that are as "universal" as possible. For example, because not all of my handguns have a slide stop that is easy, or even possible, to use to release the slide, I've adopted the "slingshot" method for reloading because it works with all of my semi-autos. I also use a "thumbs down" grip and press the trigger with the distal joint of my finder because those techniques work well for me with both revolvers and semi-autos.
You also mentioned that you shoot red dot equipped pistols weekly. While I commend you for your diligence, I don't get the opportunity to shoot nearly that often due to time, finances, distance, and life in general. Because I can't get nearly the range time in that you do, it would likely take me a lot longer to develop proficiency with a red dot. I've been shooting handguns with irons for decades and, while I may be rusty and thus not as proficient as I'd like, the basic skill is still there if somewhat atrophied. Likewise, many of my handguns are not only for my use, but for other members of my household who, unfortunately, get to the range even less frequently than I do. For these people, developing proficiency with a red dot is a skill that may never come and thus having one on a pistol that they have to use would be more hindrance than help.
Finally, as I said earlier, the majority of my handguns simply aren't practical to put a red dot on. In actuality, I only own one pistol that is cut for a red dot: my S&W M&P 2.0 10mm. Many of my handguns are older and out-of-production and modifying them to accept a red dot would significantly diminish their value and still others simply aren't capable of being so-modified (I think an RMR might actually be just a big, if not bigger, than my Beretta Jetfire or Kel-Tec P3AT). Even of the ones which I might be willing to have milled, the expense of doing so doesn't offer enough benefit to be worth it to me. I'm not trying to say that red dots aren't worthwhile for anyone, just not for me.
I have repeatedly shot handguns with red dots from 3 yd to 100 yd and been able to reliably make hits. Your uncertainty is of course your own, but I have experience doing this.
If you choose not to use a red dot that’s of course your call. I am not you and you are not me. Nothing I said above is me saying you “must” or even “should”, it’s me countering arguments that I have seen regularly (some I even held myself) that I found untrue in my own experience. The discussion is the point here.