L123
Sounds like you're doing OK. Being up front with HMRC from the start is always the best policy. Doing it after the event can be rather less comfortable!
As you say, the FI with only one customer is in a far weaker position.
For those who are interested - this is the start of HMRC's published guidance on status. There is specific guidance on individual occupations elsewhere in the manual, but FI is not dealt with.
Guide to determining status: overview - table of contents
By the way, I'm aware of one large FTO that has everyone on PAYE. My guess is that they were turned over by HMRC some years ago and are not willing to have that experience again.
ALEXA