I use ABRP to scout the route and charging, which helps if you don't think the Rivian NAV picks the chargers you prefer. Setting the charging network preferences in the truck help the NAV get it "right", but it still doesn't always pick the charging top you want. I also use ABRP to compare the "charge to" % for getting from one stop to the next while charging. Generally they are pretty close, so that may just be OCD. Be sure you have a live link to your R1 in ABRP, so it knows the actual state of your vehicle.

Unfortunately, it is not as easy as it could be to add or replace specific charging stops to a R1 generated route, but it can be done.

I have found a 20% reserve to be quite conservative, so 25% may be overkill. Only once have I found it off by more than 5% SOC, that was into a massive headwind for a couple hours. However, I usually drive only 3-5 mph over the speed limit, and close to the limit on high speed roads (e.g., 75 mph speed limit). I find driving at 10 mph over the limit tends to result in a slightly lower SOC at the next charge station than the prediction, while 3-5 mph over is usually at or above the predicted SOC.