Hi Dimitris,
Custom Investments are considered investments and depend on the investment meta object, so any customizations are to be considered. In parallel, they have investment specific modules like financial plans, status reports, etc that don't sound like being required for use this case.
So in my opinion, its an overkill not providing benefits to the use case but can lead to potential risks.
As for the custom object, yes when an instance is created the creator automatically gets instance rights.
Altogether, we are focusing this conversation on certain items but implementing a ticketing system in Clarity requires design and considerations depending on the requirements of your user community.