Generating 'Decent' numbers
Is there any room for improvement?


Yes there is.

You have a Diophantine equation \$3x + 5y = N\$, (\$3x\$ and \$5y\$ being the number of occurrences of 5 and 3 respectively. A base solution is \$x = 2N, y = -N\$. All other solutions are in form of \$x = 2N - 5k, y = -N + 3k\$. That is, you just need to find \$k\$, which makes both numbers positive. Such as k = (N-1)/3 + 1 (if x becomes negative, there are no solutions).

Bottom line is that neither the loop nor the special cases are needed.

See Wikipedia article for details.

Edit: Paper exercise as requested:

Let N = 8. Base solution: x = 16, y = -8. (16*3 - 8*5 = 48 - 40 = 8).

k = (8 - 1)/3 + 1 = 3. x' = 16 - 3*5 = 1; y' = -8 + 3*3 = 1.

3*x' + 5*x' = 3 + 5 = 8.

Edit: another exercise.

Let N = 30. Base solution: x = 60, y = -30.

k = 29/3 + 1 = 10.

x' = 60 - 5*10 = 10; y' = -30 + 3*10 = 0, yielding the decent number of 30 occurrences of 5 and 0 of 3; just as requested.