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Maybe if I'd have the money and skills to do that :)
It's unbelievably easy if you buy a pre-built wheel from somewhere like Taylors wheels. https://www.taylor-wheels.com/bike-wheels/28-inch-bike-wheel/28-inch-rear-wheels/?laufrad_hr_nabe=327
it is literally just a wheel and shifter change, then adjust the chain. (Ok if you have a double or triple crankset you might want to swap that out as well)
This is my husband's 1998 steel mtb converted to hub gear. This one is not a beauty I know, but shows it can be done as a straight switch. https://www.lfgss.com/comments/14255088/
Belt drive is not simple, but just IGH is probably the easiest switch you can make.
Buy a regular bike and then convert it to an internal hub?