As a 4.5 male, I have also abandoned 8.0 mixed.
It is not fun. It forces you to abandon traditional doubles strategy.
I agree with the poster that suggested straight up 4.0 mixed or 4.5 mixed.
I would take it one step further:
Either do not allow seniors to play 18+, or rate them down 0.5 (so a 4.0 senior plays as a 3.5 in mixed). In my experience, this applies more to the ladies. I have played against 4.5 senior ladies and they are nowhere near a 4.5 man and nowhere near a younger female 4.5.