Thinking about this one some more. What to do?

Several in the thread have survived this. I saw it. So we know it can turn out OK. Remember that. Don’t panic. Also don’t try to accomplish much. Mostly you need to rely on the inherent stability of your bike. Let your bike find that stability.

Most of what you can do is preventative. You are already on top of good tires and good inspection. That Marrakesh is made for carrying a lot of load. If loaded keep it balanced and snug. Also if loaded your 30mph is quite fast, maybe use the brakes a lot, stop to let them cool off. If it is to be a long and challenging descent consider using the manual dropper post - get off and lower the saddle. If you are not likely to do that, learn the technique of hanging your butt out behind the saddle on the steep stuff. It is way more stable that way. However you go downhill ride within your capabilities. There are days for pushing the envelope and most days safety first. Only push the envelope when the bike is unladen, the weather is perfect, you feel fresh and fantastic. 30mph is not all that fast, a lot of us do that on the flat, or did when young. Most of us can do it on little tiny hills. Downhill crashes are always worse, the likely case is still only you break a collarbone. Which heals quick and is not that big a deal. Whatever you do avoid high speed crashes in traffic. If such a thought worries you, slow down.

When you slow down remember that prolonged heavy braking heats the rim and will contribute to the blowout you were hoping to prevent. Stop once in a while and enjoy the view while the rims cool. Feel the rim and get an idea what sort of braking does this. Discs? They overheat too.