I think the short answer is that if you're running a configuration where
the requested fulls just barely fit before the tapes cycle, then if tons
of data is added then yes it will overflow.  This is pretty much
inherent to using that many tapes.

For reliability, you should have more than one full of everything on
tape at all times.  So if you are going to have the normal weekday
runspercycle 5 and dumpcycle 7, you should have probably 15 tapes, or at
least 12.  And, your tapes should be at least twice as big as what
"balance" outputs.  When all goes well, you'll be just overwriting the
third full dump with each new one, leaving two.

In my view, tapes are cheap (even LTO-2 are $32 each), compared to the
value of data, the cost of the tape drive, and management time, and it's
silly to try to run with fewer tapes than is appropriate, especially
only a handful.

I would agree that amanda is not optimized for operation with too few
tapes, but I think it does pretty well given in what is essentially an
untenable situation.