Ceremony at church followed by 2 receptions?

Hi, Chris! This is what's called a tiered wedding, which is usually considered rude. I personally feel it's okay to have a private family-only dinner after, but nothing extra like a DJ

View Quoted Comment This.
Plus I feel like it gets complicated tracking who should be at which event. You could just stick with the cake and punch reception and have the family dinner as the rehearsal dinner.

I think it could get awkward at the 1st Reception when people start talking and realize that there's a second event going on that not everyone is invited to. I know I'd feel uncomfortable if I mentioned the upcoming dinner and the person I was chatting with had no idea what I was talking about. In fact, you may get a guest or 2 who says, "Oh, Yes, There's a dinner after. Didn't you know? It's at 3pm!" or something similar. That could get very awkward, especially if they show and you have to turn them away.

In addition, if I was one of the 50 included, I'd find it odd to be cutting the cake before having dinner.

I think doing it that way might be off-putting to your guests.

I would find this odd and rude. Also for an 11am wedding I would expect to be fed a full lunch afterwards as that is lunch time so I don’t think that’s a good time frame for a cake and punch reception of any sort.

I don't think this is a good idea, to be honest. Just like pp Theresa mentioned, imagine at the first reception talking to another guest about the "dinner" coming up, and then not knowing what they were talking about. Lots of people could potentially feel very slighted. Is there any way to evaluate these 100 guests and shorten the list to those you really want there? And then just have this number of people come to your ceremony and your reception, and not just half go and half have to go home after the cake cutting. It's not a good set up, what your future wife has proposed, to be honest. I understand the financial part of it, but it will hurt people's feelings, no doubt about that. Hopefully she will reconsider and think about the cons of her approach.

Not a great move unfortunately, for all the reasons previous posters state.
If this helps, we are having a small, intimate wedding, its our immediate families and 4 other families who are close to us, with a few stragglers. Comes out to around 60 people. About a month later, my future mother in law is looking to host a potluck for us. Not a wedding, no gifts, no wedding things, doesn't matter if you show up in white or in jeans. Just a fun party, bring a side if you want & we'll have the main meal there, a fun band, and some lawn games. Maybe bring up that sort of structure to your fiance? If it helps for numbers, we are in a particularly expensive wedding area, our 60 person wedding will be between 8-10k (not cutting corners, I definitely have some splurge spots, if you're looking to save you can for sure trim that down!) and the potluck with be at a free venue that doesn't allow drinks (a church) so the cost is relatively low for that, pretty much just the band, a main food and some extra sides just incase, and whatever decorations my future mother in law decides she wants. We estimate around 1.3, 1.5k for it.

Since they are in the same day, this would be considered rude. Only half of your guests are truly being hosted. I'd be hurt to hear I wasn't invited to the actual reception later in the day. You can invite less people to the ceremony, but not the other way around.