I've been interested in Mullet drive trains for a while, however I don't know much about Shimano Mullets so i'll be interested to see how you get on and what parts you've needed.
I'm running SRAM Rival 1 on my gravel bike with a 10 - 42 cassette and as I found out to my cost, running a stock 36 tooth chainring was really not conducive to loaded hill climbing. I'm currently running a 32t chainring, which to be fair, gives me a more than adequate gear range most of the year. When I go touring though, I do swap it out for a 28t chainring in order to get up the hills, but obviously then sacrifice a bit of top end speed. I really don't want to change the bike to 2x though, not only do I really like 1x, but fitting a front derailleur would comprise my ability to run the fat tires I need for the terrain around here.
Intriguingly, whilst many mulllet options exist for SRAM, it turns out there is actually a simple and "cheap" way. SRAM rear mechs have a plastic cable fin that the cable wraps around before going into the retaining bolt. Swapping the cable fin from a road derailleur (Rival or above) onto a mountain bike derailleur (Eagle GX or above), changes the pull ratio, so that SRAM road levers will actuate a mountain bike derailleur. This then gives the option of 10 - 50 cassettes on the bike. The problem with all of this, is that SRAM don't sell the cable fin as a spare part, so you need two very expensive derailleurs to swap the parts around on.
Even better still, Ratio parts in Britain, offer a 12 speed conversion kit, so I can could go super wide range with a 12 speed set up, however I just can't justify the expense for the few times I really need the extra wide range, as stated above, most of my riding needs are met by my current set up and swapping the chainring is cheap and easy to give me the taller gears for when I go touring.