I just try to get as much gamerscore as I can and when the expansions come out then I go for the millennium mark, go for easy achievements and aim to complete the 3,000 Forza score for each game if I can.
I'd try to complete all of them if it wasn't for me having one heck of a high stress personality.
I don't think there are any that are particularly difficult in FH4, FM7's were harder for sure. I'm pretty sure my rewards thing is broken anyway as I have zero ultimate passes and it says FM7 has 30 showcases when I count 21 (?), and as I'm on PC if I got every single possible point I'd only just be tier 8 so I'm happy with settling for 7. I only need like 200 points now.