I'm a new motorcycle rider (8 months, 5000 miles, plenty of rain; 30 years car driver). All your common sense thoughts are probably right. (For instance: ease onto the brakes, don't jerk; ride slower; lean more gently so you don't undermine your traction; etc.) But I'd add one more:
Rain aside, physics keeps a motorcycle upright when it's moving (someone else can explain it, but you've probably noticed that tight maneuvering is harder when you're going slower). When you slow down for rain, be aware that if you slow down TOO much, physics will no longer be helping you.
The only time I ever took a spill on my bike was when I slowed down TOO much when riding over a rain-slicked steel plate in the street. The side-slippery forces became stronger than the bike's forward motion, and the bike slipped out from under me. Specifically: the rear wheel started sliding to the right, and then the bike fell toward the left and spiraled forward counterclockwise on the ground out from under me. Neither I nor the bike was seriously hurt but it was really embarrassing.
Since then I have taken extra care to proceed steadily and loosely (not too tight a grip) in slippery weather, as well as slowly.
But don't psych yourself out. A bike in good repair, with good tires, handled responsibly, can ride adequately in the rain. Just practice in low-traffic conditions, and be attentive.