It is very difficult to give advice about something I/we can’t see and as I have no experience with the developer you mention and no idea about the original lighting range, I may be wrong. However, the fact that you mention there is a bit more grain than expected, I would guess you have over developed the negs slightly. The good news is you are using a (cold light) imagine this to be a diffuser which should help. I would suggest the ideal is to expose and develop for the enlarger system you are using.