You are a really good game plan going

I'm a 4.5 getting ready to play at 8.0 Nationals later this year. My partner has been coming through the ranks from the 2.5s (I believe just like you), and she's still getting a lot of coaching to this day.
This was her first year playing 8.0, and when I first partnered with her, she had the natural tendency to close the center and reach for balls there. That would be the right strategy in 3.5 women's doubles, but in mixed, it makes the situation a bit difficult because just reaching to put the ball back in play is not good enough, especially against strong pairings. now at best, you find yourself on the defensive when you probably had the edge in the rally until that point. Moreover, you leave the alley open, and the gap feels larger than it is to the opposing guy, since he doesn't feel the pressure from dealing with a world class volleyer.
Since then, I've asked her not only to stand a lot closer to the alley than she regularly does in women's, but also a lot closer to the net. That way if the guy wails the shot at her, even if she gets a frame on it, it could still go in. I don't serve and volley systematically as a result, since I might need to cover for the lob return. I ask her to stand a lot closer to the net than normal when I return as well, and close in right away unless my return is a floating lob, as the top pairings will very most likely try to go back to her right away.
When you are serving, yes, try never to double fault, and sometimes just putting a soft ball is the best serve. It gives your partner more time to fake or poach, and more time to your opponents to think about the return and miss...

They might be able to wail the returns and hit winners at the start of the match, but they might not be able to do the same late in the set if the score is close.
When she returns, I'm often standing back, especially on the guy's serve, but also on the women's serve if the guy is a heavy poacher. If things aren't going well and you feel the court is too small on your returns, then perhaps you could suggest that to him.
Also, some people joke about the 3.5 women just putting her serve in and then going to sit on the bench...

I haven't really tried that strategy to be honest, but I know my partner has been able to come out with great shots in clutch moments in the many close matches we've had so far. I'm not sure if she would have been able to do that if I had been overreaching for balls all day long, as I think it would have made her tighter as the match goes on.
In general, I feel attitude is one of the most important factor. you might end up being the obvious weakest player on the court if you are facing a 4.0/4.0 pairing, but your partner will understand your limitations. After all, he has limitations of his own as well. Don't get discouraged, nor intimidated, keep on trying your best. You might go through a spell where you feel you are completely over exposed and overmatched, only to hit a great shot at a clutch moment. Hopefully, your partner will fully recognize and acknowledge that when it happens, and try to keep the momentum going.
Good Luck!