OP, this is where you need to go with something very simple that looks impressive. You are stressed and unhappy about cooking (for very good reason) and don't feel confident.
In this situation, I agree that chicken is not a good option (outside, possibly, of a tray bake). It's too easy to stuff it up.
I would go with beef. A large rib of beef (for 6 people I'd go with 2 ribs). You could go for a fancy sirloin rib but I'd go for the bog standard rib as it is insanely forgiving. You calculate the cooking time according to this website But, and here's the genius part.... if your oven is a bit hot/cold or you cook it for a bit too long/ too short... it doesn't matter.
Then, as it's summery you want, don't do a full roast with all the trimmings. Too much time and effort. Plus, this way you can serve the meat warm/room temp and just leave it to rest for whatever time is necessary while you're doing other things.
Instead, go for a summer version with salads. Last time I did this I did buttered new potatoes, a mixed colour tomato salad and a lovely big fresh green salad. I also made a "green sauce" (chopped herbs, capers, garlic, oil) for the meat which I served sliced on a platter.
Other salad/side options I would consider would be roasted veg and couscous, griddled aubergine with a tahini dressing, some kind of mezze style bean dish (I did the ottolenghi beans with green tahini the other day and it was very nice), a really good slaw (without mayo I'd think but that's up to you) or something with mushrooms. But frankly, whatever salads you like, would be fab, I'm sure.
That's my 2 cents worth.