Casual engagement party recipes for 20 +?

Check out this contest winner recipe for Your Best Cheap Feast. Read the comments, too for variations. https://food52.com/recipes/21102-greek-lamb-with-orzo

Everyone loves a great deviled egg. Make them special with a bit of bacon or roe on top.

If someone asks if they can bring something, say yes! Ask them what they want to make or if they know of something that they make that the bride and groom to be love.
I've never offered to help if I didn't actually expect someone to say yes.
Take advantage of these mutual friends. Just make sure you know what they are bringing so you don't have a million desserts and only 1 app.

Cava is a perfect choice for beverage on this festive occasion.
Here are a few easy recipes I have often used for large groups. The Duck Crostini https://food52.com/recipes/25199-duck-liver-crostini can also be made with chicken livers; the Spinach Balls can be made ahead of time https://food52.com/recipes/7160-spinach-balls-with-mustard-sauce; and the ribs are so easy and go so well with bubbly... https://food52.com/recipes/13872-1-2-3-4-spareribs. (Only problem I foresee with the popcorn is you are likely to find it ALL over your house after the party.)

I agree that you need to be sure to have something a bit more substantial, and I agree that a spiral ham is a great way to go. For some other ideas . . . . I love to make focaccia and top it with different things (tomatoes and garlic, onions and olives). Gougeres are easy and elegant. You could do brochettes (little skewers) of marinated chicken and beef with or without a dipping sauce. You could take fancy toothpicks and make Caprese salad bites -- a grape tomato, haved, a piece of basil and a small cube of good, fresh mozzarella. Another vegetarian idea is to take a piece of endive, and put some blue cheese crumbles and candied pecans in the endive "boat" and then drizzle it with a balsamic vinaigrette.

If I was invited to a casual engagement party at 5 pm I would expect dinner unless the invitation specified cocktail party. I would strongly suggest anything substantial besides booze and nibbles. Sliced spiral ham with biscuits, skirt steak with rolls, grilled chicken with pitas. A buffet of a variety of mini sandwiches.

Thanks, team! This is all great advice. Especially love the cava/popcorn idea! If only...

is it within possibility to get a co-host and share the hosting responsibility...

Roasted vegetable platters can be relatively inexpensive and delicious at room temperature, which is a plus: zucchini, eggplant, peppers, etc...you can leave them out with some baguette slices and oil and vinegar for drizzling. And cheese, if your budget allows for it.

The relish in this recipe would be a delicious accompaniment to your heirloom tomatoes, and easy to scale up if necessary:

http://www.foodandwine.com/recipes/green-tomatoes-with-pistachio-relish

I LOVE your idea of radishes with butter (perhaps with a few flakes of Maldon), so elegant and so simple.

For dessert, crème fraîche can be relatively expensive, but I've softly whipped sour cream with heavy cream (and a touch of honey) and approximated it. You could bake meringue kisses (infused, even) for a bit of texture with the fruit. Carton egg whites are inexpensive and I use them often. I've done mini malted-buttermilk biscuits with fruit and cream for a sit-down dinner once. Even in an elegant setting, rustic food is often welcome, especially when its a smaller portion. Profiteroles with caramel and pastry cream, sponge cake petit fours with fruit, madeleines with tea infused panna cotta (a Proust inspired dessert), sablé/shortbread with dipping chocolate, canelé. I've done elegant, homemade "Hostess" cupcakes and twinkies before, that were a hit.

All over the place, but those are things I've done in similar settings with rave reviews.

Take a look at the entries from the One-Bite Party Snack contest here:
https://food52.com/contests/265_your_best_onebite_party_snack?logged_in=false&page=1&uid=21631