I have hundreds of cookery books, but in warm weather often default to several recipes from Delia Smith's Summer Collection, which are regular favourites. I know some foodies can be a bit snobby about her - but I don't care!! I used to invest a lot of time & effort in cooking for dinner parties, but just can't be bothered these days, & these are all simple & lovely.
Personally I wouldn't bother with a starter - maybe have some marinated olives/roasted almonds (good bought or homemade) as nibbles to accompany drinks when people arrive. And do something summery like an elderflower cocktail or spritz as the first drink? And then serve all the mains/sides/salads at once & let people help themselves?
These first two make lovely starters separately or together, if you DO want to do a starter (you'll need bread on the side), but I would just do as part of the main spread, personally:
https://www.deliaonline.com/recipes/collections/delias-summer-collection/fried-halloumi-cheese-with-lime-and-caper-vinaigrette
For veggies you can swap the anchovies for black olives/capers - https://www.deliaonline.com/recipes/collections/delias-summer-collection/piedmont-roasted-peppers
I love this fish dish from her, & make it often - though I use salmon fillets not steaks - no bones! https://www.deliaonline.com/recipes/occasions/easter/easter-recipes-using-seasonal-ingredients/salmon-steaks-with-avocado-and-creme-fraiche-sauce
And this alongside it - https://www.deliaonline.com/recipes/collections/delias-summer-collection/warm-potato-salad-with-lemon-and-chive-vinaigrette
As an alternative cheese dish to the halloumi, this is an ancient & basic Nigel Slater recipe which is done in the oven, so you could cook at the same time as the fish, or after when the salmon is cooling. I pimp it up with thin slices of red onion, & some sliced jarred peppers if I have them - & in summer if my thyme or oregano is flowering I add the flowers & it looks SO pretty! https://wtfdoieattonight.com/2011/07/15/nigel-slaters-baked-feta-with-thyme/
I also love Diana Henry & her summer recipes are always great & uncomplicated. This tomato tart (which I make rectangular not round as it's easier with bought puff pastry blocks!) is delicious -https://eu.palmbeachpost.com/story/entertainment/dining/2017/06/06/recipe-week-fragrant-cheese-filled/7172545007/
Tabbouleh (properly made with scant bulgur & copious parsley) would be another good dish, plus (if you don't do the tomato tart) a tomato & basil salad using all those pretty different colours of tomatoes in the shops now, &/or a simple green salad (I love butterhead lettuce in a classic French vinaigrette).
Favourite & very easy summer desserts include this Nigella one for baked plums with Barbados cream that you make the day before: https://www.theguardian.com/theobserver/2000/mar/19/life1.lifemagazine4
I also do a baked peach dessert with amaretti & ricotta & honey, but it is such an ancient Nigel Slater one I can't find the recipe online!
Failing that, a monstrously huge bowl of Eton mess is always lovely ...