Fed Chair Powell has testified that the Fed expects to keep interest rates at near-zero levels until 2023. However, if inflation did appear in some form before this, he would change his mind and raise rates. The Fed has also said it would let inflation overshoot their 2% target before acting, so they will be chasing it rather than trying to predict it. I read this to mean that whenever it will happen, it will happen fast--perhaps faster than processes like mortgage refinance can react to.
You mention taking out a 30-year mortgage, but how long would you intend to keep it? If you are the type for whom paying off a mortgage early is a dumb move, then I would suggest taking one now and locking in historically low rates. To me, optimizing your payments for 3 years doesn't outweigh the assurance of minimum costs over 30 years. You might play with the ARM for a couple years, based on the Fed's current position, but then you're in the futile business of predicting the Fed.
If you are the type who would pay it off early anyway, then you might lower your rate with a 15-year fixed loan. It would still be dumb to optimize the first 3 years of that, a the cost of the last 12.
I also worry that the timeframe you are analyzing is the blink of an eye when considering housing. My first mortgage in 1996 was at 8%. I was thrilled to refinance at 6%. The numbers now are not normal, and there is nothing that says global finance has moved so that those more-rational numbers can't happen again. How happy will you be chasing the current spread for 6 years, vs. how sad you would be to be paying historically average rates?