inrelation to your current scenario, SEPA direct debit, is quite unpopular highrisk processors adopting such processing channel, would ideally be registered/operational and catering to demographics within the EU/SEPA Area.
probably initiate a research/database of highrisk processors country by country within the Sepa zone and then narrow down to the specific services you currently provide.
Why do you see a smaller CB for credit/debit cards?
Currently capable and competent highrisk processors operate strictly within the 3Ds,securecode,PCI,L1SP modules and integration, which filters out genuine card Bins,restricted bins,Card variants etc at the point of client card payment at checkout cart.
the above parameters allow for only genuine cards(credit/debit) payments and onwards processing of such cards into merchant acct, and then final phase of payout/settlements.
fraudlent,dodgy cards,prepaid card,restricted bins etc would be filtered out at the point of checkout cart, thereby reducing the overall CB ratio based on payments made by cards.
with the above parameters in place, practical cases ie, CB issues would only arise from direct customer complaints, based on non delivery of services,delivery of a non functional item, shipping delays,low quality products/services etc.
rather than reoccuring CB ratios, CB complaints, and processing restrictions based on card payments.
A visit to The MG resources section would be Of Utmost Value, towards guiding as well as directing you towards your precise scenario...