The real trick is to stop treating dietary restrictions as individual requests and bundle them into actual dish categories that work. Ask your caterer what they're genuinely confident making in volume - like, can they do a solid gluten-free main? A vegan option? Something shellfish-free but otherwise flexible? Once you know their actual capability (not just "yeah we can do anything"), you build your menu around maybe 3-4 solid options plus a safe default, then tell guests to pick one. This way the caterer knows exactly what they're prepping instead of juggling fifteen special snowflake requests.
The pitfall everyone misses is that caterers will say yes to anything just to book you, then cut corners or mess up because they're overwhelmed. You need to specifically ask them: "How many gluten-free plates can you reliably serve?" Get them to commit to numbers. If they say 50 but you're guessing you'll have 40 dietary restrictions, that's fine - they've got cushion. If they hedge and say "well, maybe 30-40 depending on the day," that's a red flag they're not set up for it.
When you send the guest form, make it dead simple: "Choose one - Option A (our main), Option B (vegetarian), Option C (gluten-free), Option D (vegan)." Most people will pick A or B. You'll get actual numbers in real time, and you can confirm with the caterer a week before with exact counts. No surprises, no insanity.