21 million is plenty enough to be rich, i wouldn't worry about it.
you seem pretty big by now so it doesn't seem that you need to worry that much about generating cash but nevertheless.
if youve got some good players who are reaching the latter part of their carreers (29+) they should be pretty highly valued. if you can sell them off for purely monthly installmant fees (12 or 24 months is usually best) you can turn your monthly turnover profit on its head.
ive found this best for club growth. like for moving a boro onto a liverpools playing feild which is a big step in the world of modern football.
i had to continuelly do this to my best players to eventually get barnet into the
prem financially secure which took a few years but was worth it.
but dont go round selling your best players if you don't need to!
once your really rich you can only seem to get richer even by signing big players etc.
good luck