Getting a tire off the rim - 37-622
Counterintuitively, one pump of air in the tube may help separate the two beads of the tyre, allowing you to grab just one with the tip of your tyre lever.
I have a bunch of tyre levers, and some are good while others are dreadful.    See if you have one with a flattish front, not a spoon or bill shape.
It can help to use some cable ties or velcro straps to squash the tyre into the rim's internal valley.  Then work your way around the tyre pushing the whole thing into the rim whilst moving the excess around the circumference to the opposite side.  This is generally how one would fit a new tight tyre like a Marathon.  You're doing the same process in the same order but aiming to get the bead off.
Last resort, if the tyre's dead you can cut it off but this means the tyre can't be reused,