Thanks for the response. My goal is to have the Topstone with an 11-40 or 42 in the rear, but stick with the 105 model given the Shimano components I want. I kind of want to avoid the Apex 1 model.
So the RX812 RD won't fit with the stock cranks on the 105 Topstone?
Thanks again for the help.
No, because the RX812 is for 1x only. The Topstone has two chainrings at the front, a 46 tooth and a 30 tooth. This is to provide a wider range of gearing over a narrower cassette, like an 11-34. On the other hand, 1x gravel drivetrains have a front chainring of 40 teeth or 42 teeth. The range is made up for by having a wider cassette in the back, such as an 11-40 or 11-42.
Combining the 46-30 chainrings with an 11-42 cassette gives you lowest gear of 30-42, which at a cadence of 60rpm, gives a speed of about 5.7km/h. For most people, that's slower than walking pace. If you're regularly encountering territory that requires that kind of gearing, then a mountain bike would be much better suited to the task as it maintains stability significantly better at slower speeds.
There's not much you won't be able to get up with a 46/30 and 11-34 combination. Try it first.