It seems that your indications regarding changing conditions following sea level rise are valid. Rising base level will move the current depositional processes inland some distance as you have indicated.
The process will apply basically in the same way to all rivers that terminate in a body of water that will be affected by sea level rise, for example rivers that terminate in a lake that will not rise due to the changing climate will not be affected. There are some circumstances where lakes in steep terrain could be affected by increased meltwater from surrounding glaciers, and that increase could be significant, but the models predicting sea level rise are not referring to particular local conditions, which are on a case by case basis, but possibly very important to the local populations.
Where the process becomes interesting is what happens to a particular river, because they will all be different. In order to determine what will likely happen, one must consider the physical aspects of the river, what is the change in elevation near the terminus, what is the topography of the floodplain, how much does the river meander as it approaches the terminus? These and other similar factors will determine what happens as the base level rises.
If you consider a river that has very little change in elevation, and meanders across a wide floodplain, the amount of inundation caused by a one meter rise in base level will cover a large area of what is now floodplain and likely intermittently wet or dry based on seasonal fluctuations. Compare this with a river that occupies a canyon, or is deeply incised and has a steep gradient to the base level, the base level will rise within those constraining features and little will likely happen to the narrow floodplain that has been present.
First step for your inquiry about the Rhine River is to look at the topography surrounding the river, and mark out a one meter change in the area surrounding the river beginning from the terminus, as the terrain becomes steeper toward the head water the change from current condition will diminish.
Direct answer to your question is that the inland affect of one meter of base level rise could reach much farther than one meter upstream depending on the topographical features near the river terminus, and each river will be different.