Personally, I don't consider being dropped the ball on or left with fewer options - I currently own 2 RST-300 mounts and a 135 each for different location and purpose.
Unlike the RST-300, the RST-135 model is slightly older and the price & capacity is in the range that more hobbyists are interested in. As such, it's bound to have more adoption and more options readily available off the shelf. As you go higher priced / capacity mounts, you are inching your way into nicheness be it Rainbow or any other manufacturer. I have experience with other manufacturers and this is no different.
Like I mentioned on the RainbowAstro mailing-list, the Mach1 plate is quite useful because there are so many mounting options once you have Mach1 compatibility on the RST-300, starting from permanent piers to tripods. People who are shopping in this catagory (like you and I) would likely have invested in a proper pier/tripod so it in fact makes more sense to let the customer choose whatever works best for them.
For the sake of completeness for folks looking at this thread in future, I'd also vouch for the Supermount CYG54G tripod if you are after a travel-only setup with the RST-300 (no Mach1 adapter needed).
Edited by base16, 18 September 2021 - 02:18 PM.