I think Leatherworker.net is a good website to get information about heavier sewing machines. Perhaps you like this information:
https://leatherworker.net/forum/topi...o-sew-leather/
If you want the machine to sew leather, then you will like the machine to be able to sew slow in a very controlled way and it should not jumpstart to some higher speed.
Some of the vintage domestic sewing machines can with some modifications do some heavier work for some time, but it is my guess that you should not expect above 500 hours of work. Such machines are often cheep to get, and you will need to cope with trouble for some time. This is a modified Singer 201k sewing 2x10oz leather:
https://youtu.be/HIDuHjMRFWg