I want something small that will throw a golfball or tennis ball an
impressive distance. So I was thinking maybe a "King Arthur" style
trebuchet (with a fixed axle, not floating.)

Has anyone here built something like that? Maybe I'm at the wrong
group, but I don't think so.

How important is it to be able to adjust the amount of counterweight? I
was planning to use a sledgehammer for the hanger arm and counterweight.
Or maybe a beer can sized block of lead -- about 8 to 10 pounds.

I assume the throwing arm needs to be as light as possible, especially
towards the end.

No idea how one adjusts the sling.

Best regards,
Bob