I have multiple category of clients:
Installment payment provider
Startup/Project Financier or Investor
I am looking for an umbrella word suitable to substitute the above in general.
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It sounds like these entities are all creditors in your situation, as opposed to the other clients who are debtors.
They are all economic actors The Free Dictionary says an economic actor is:
A person or unit able to use land, labor or capital. An economic actor uses these resources to shape an economy, usually (though not always) for his own benefit. An economic actor may be an individual, a company, a government or even a society as a whole.