Dynalmadman,
I just went through something similar, trying to sort out the best options for aging photographers (me).
I have an R6 II arriving Wednesday, so take this with a grain of salt, but from what I have read so far, Canon's IBIS works with lens-based IBIS with both RF and EF lenses. I agree: I wouldn't buy EF lenses now.
Re focal length: the IBIS in the R6 II adjusts for focal length. In fact, if you attach a lens it doesn't recognize, it lets you specify the focal length.
You haven't said what the alternative to the R6 II would be for you. I have the 5D mark IV, which lacks IBIS. The R6 II is a big step forward in several respects in addition to IBIS, although not knowing what you photograph, I don't know whether they will be of benefit to you. In particular, the AF is vastly better and more flexible, which will be useful for me.
Re weight: my unsolicited advice is that if you are concerned about weight, don't buy f/2.8 lenses. They are far heavier than f/4, often literally twice as heavy, as well as being far more expensive. All that for one stop. Unless you need the narrower depth of field from the extra stop, the f/4 will do just fine, and Canon makes very high quality f/4 lenses. The R6 II handles low light very well, so in the rare case where the extra stop would be useful because of low light, just bump ISO up by a stop. I made the decision to buy f/4 rather than f/2.8 telephotos years ago, when I could handle weight better, and I virtually never regretted it. With the R6 II, which handles low light better than any camera I've had before, I expect never to regret having f/4 zooms.