Hi Op sounds great to me! You sound very hospitable bc I always put a closing time on the invitation eg 7 to midnight nowadays so people don’t stay for hours and hours but I am getting old I guess! 😄
After what seems like a life time of hosting the things I would suggest are:
A couple of nice big bowls of non-leafy salads go well with barbecued main item eg a big cous cous salad with roasted veg or a big bowl of asparagus salad with mixed grains (or carrot salad is more budget friendly) and they keep better and are handled and eaten more easily by people who are standing up while eating rather than large green salad leaves. They also keep fresher for longer and don’t wilt. A bamboo spork for guests is useful.
We tend to go for beef kebabs for guests that are pre-marinaded and cook more evenly on the barbecue than burgers or sausages.
It’s great you have one ft dedicated person on the barbecue who doesn’t get drunk or distracted as that is essential 😆
Also have a couple of helper friends dedicated to serving drinks, attending to the less mobile guests, answering doors, clearing bottles, collecting rubbish, finding coats etc, as you can’t be in all places at once.
But best to provide as many chairs and cushions and picnic rugs for the younger guests as possible; you can hire stackable chairs quite cheaply for the less mobile ones.
Do you have a plan B for inclement weather eg an emergency canopy or something?
I always fill up plastic garden trugs full of ice, or a clean wheelbarrow, attach a bottle opener to the handle with long string or thin elastic and and place beer bottles in one and water bottles and soft drinks in another for people to help themselves.
Put out lined plastic bins well signed for rubbish already separated in to plastic, paper cups etc.
Always serve far more veggie dishes than meat dishes as the omnivores eat both!
Arrange your dishes on one end of one table and have clear space for dishes brought by other people along with plenty of serving utensils.
Completely agree with wording that a pp suggested; eg we are serving this but any complementary savoury dishes welcome! That stops people bringing too huge a variety of things.
I tend to always keep control of puddings but you can ask people to just bring those and hope for the best!
I tend to bake meringue rings a couple of days in advance and keep them well wrapped up in Tupperware. Then prepare whipped double cream the day before with icing sugar and a bit of alcohol 😃. Then wash and prep berries the morning of the party and assemble last thing!
Make a load of mango or citrus sorbets and put in freezer as an alternative.
Make sure the lighting is right in your garden way in advance with string lights for atmosphere and safe lanterns etc but make sure wobbly slabs and steps are properly lit as it is guaranteed that someone will fall down one if not while talking, drinking etc and you are liable if so!
Provide blankets and throws for when it gets chilly later on.
Strongly advise do NOT offer tea or coffee but offer one thing to finish; or you will be at it all night! A hot toddy or an Irish coffee or mint tea but just one choice!
Always remember to put cutlery, napkins and sauces at end of food tables rather than at the beginning as they are easier to juggle that way.
Have the number of a taxi company prominently displayed for anyone who has over done it!
Have fun!