In our org we have a custom object which is linked to account called spend account, this has a one to many relationships with accounts, Marketing sign up contacts to campaigns but also need to know all related spend accounts to sign them up on our ERP, below is the current setup:

1 account record can have many spend accounts related to it
1 account record can have many contacts related to it
1 contact can have 1 spend account linked via a lookup field (This causes problems when the contact's account has more than 1 spend account)

A contact is signed up to a campaign and Marketing report on the account records the contacts are on however they need to also know all the related spend accounts the account has (currently they run a report to show all the spend accounts related to the accounts of the contact but this is a manual process).

In a sandbox I have added a junction object between Spend Account and Contact which allows a contact to have many spend accounts related to it, however, I'm looking for a flow/process which would automatically update the contacts related spends to be the same as the account's related spends. Not sure if the junction object is the right route for this so any other ideas or help on how to achieve this would be gratefully received.

Many Thanks.