Moving to SEPA functionality provided by SAP in standard is a huge challenge for many customers.
It is clear that SAP ERP has a higher priority in this context than SAP CRM. So it is often a question if and when it makes sense to also move SAP CRM to the SEPA functionality provided by SAP in standard.
I would like to provide you some hints that hopefully make this decision a little bit easier.
The main question you need to answer is: Do we actually need SEPA mandates in SAP CRM? Really in SAP CRM, directly integrated, for example in the bank data of the account, stored and integrated with SAP ERP via data replication.
If you only need to be able to maintain SEPA mandates, in the CRM WebClient UI, - only the UI aspect is important here -, then you are possibly okay with having SEPA mandates in SAP ERP only but integrating the maintenance via the transaction launcher. This probably means own effort on your side, but overall it could be at least a first option for you.
As soon as you need to consider SEPA mandates also in business transactions replicated between SAP CRM and SAP ERP you have no real choice any more. In this case I would recommend to better move to SEPA functionality provided by SAP in standard, as soon as possible. You need such a tight integration of SEPA mandates in your CRM processes that trying to adapt on your own only a little bit can lead to more difficulties and more effort than expected.
An example for such a case is: Use of a SEPA mandate in a service contract.
You can find the details about SEPA in SAP CRM in SAP Note 1828238.