You can add as many payment processors as you'd like and tie those to unique contribution pages: https://docs.civicrm.org/sysadmin/en/latest/setup/payment-processors/#setup-and-configuration.
However, the real question is reporting requirements. What are they? Are these non-profits all under a parent non-profit? If not, then they should have their own ...
There is a fix (work around). It is only needed for users on Joomla. And that is the java fix- Go to System Settings / Resource URLS. Change the path for CiviCRM Resource URL to be /administrator/components/com_civicrm/civicrm/
We did build UK Direct Debit [with the mandate - the entire workflow] into the iATS Payments Extension but it wasn't used much and we have not actively supported it recently. And SEPA works well and is widely adopted by lots of European CiviCRM Clients. One does not need to be exclusive -> you can processing CC with iATS Payments and use SEPA for DD for the ...
This is not the answer you want - but should help you get the answer you want.
Powerbase via PtP apparently has a solution for echeck via Authorize.net with Dharma Merchant Services
They may have an answer for you.