I at last found a source of cycling shoe buckles on aliexpress.
Skate Buckles by
Timothy Takemoto, on Flickr
They had more pull than the buckles that they replaced. Replacement cycling shoe buckles cost 20USD a pair. These were only about 3USD a pair including straps (the straps alone are even cheaper but did not work with Shimano buckles). The only problem is that since they are bigger it was necessary to remove the flange that guides the strap into the buckle so that these larger buckles can be used. I removed the flanges on my shoes with an angle grinder.
Alas one of the buckles broke on the third outing. The is a slight recession behind the hole where the buckle bolts to the shoe, which deformed the buckle a little which resulted in one of the ratchet springs coming loose. I think I can still just about use the buckle after cutting a bit of its plastic off (so that the dislodged spring does not get in the way, and the other spring can still tension). In future I will not tension the bolt as much and I could use a washer to fill the recession behind the buckle but, it also shows, in addition to their being too big, that the buckles are not entirely suitable for uses other than that which they were designed -- roller skate buckles. Still, one can get old shoes like this for about the price that Shimano is charging for the buckles, so using these with a washer or some other method of filling behind-buckle-holes (epoxy?) may still be a good idea.
Perhaps the other style would be better. I got them from aliexpress
https://www.aliexpress.com/item/1005...38274c4dwtm1nk