Hi,
I need to create a document library where we can store our controlled documents with the following additional columns like Fiscal Year, Team, Document Type and Sequence Number.
The fields except for the Sequence Number field will be manually filled up by the user however I want the Sequence Number field to be automatically filled up so the user won't have to look at the current contents of the library to determine what should be the sequence number of the file. This Sequence number cannot be based on the ID field because the number will depend on the other fields I mentioned earlier. For example, there could be many documents that has the same sequence number, 001 for example, because they are the first one from a specific combination of Fiscal Year, Team and Document Type. See example below:

I was able to automate the sequence number field using a flow that triggers when a document is created/uploaded however this won't work for a library that requires check-in/out because the flow can't edit a checked out item (at least as far as I know).
How can I implement this? Are there other means aside from Flow? PS: Workflows are currently not an option.