Recipe websites for bulk/restaurant quantities
I have a few cookbooks for caterers that have this sort of thing.  I've even gotten one from our library system (although not my local branch, I was at one of the larger ones)

So, using that term, I tried searching for 'catering recipe websites' had a bunch of false positives, but it did find : 


http://lotsofinfo.tripod.com/ ("Over 500 Make ahead or Large Recipes and MANY Self Catering Tips Below  --- enjoy").


A search for 'catering recipes' was a bit more helpful:


http://recipehut.homestead.com/Bulk.html
http://www.party-recipes-and-ideas.com/
http://www.customcatering.net/recipes.html
https://www.bbcgoodfood.com/recipes/collection/feed-crowd  (although 'crowd' is 8-10, it looks like)
http://www.bigoven.com/recipes/catering/best (has options to set the number of people to serve ... but that's not going to tell you how to adjust cooking times for larger pan sizes, etc)
http://www.foodnetwork.co.uk/recipe/catering-for-large-numbers-keyword--italian-cuisine.html ('large numbers' seems to be 4, but there are some that are 12 ... which might fall into 'entertaining', I guess)


I'm actually pretty disappointed in most of these -- I'd have expected recipes for 'catering' to talk about what parts you can do in advance vs. morning of vs. last minute.

There are also websites w/ tips for caterers, eg. https://www.webstaurantstore.com/article/26/catering-menu-ideas.html

I also found a website for 'Essentially Catering Magazine', but many of the recipes they have serve less than 8 people : https://www.essentiallycatering.co.uk/recipes.php

I suspect that cookbooks might be a safer bet -- many of these have the 'look inside' option on Amazon, so you can see if they're what you're looking for:


https://smile.amazon.com/Armed-Forces-Recipe-Service-Cookbook/dp/0615862683/
https://smile.amazon.com/Cook-like-Caterer-Entertaining-Catering/dp/1475019017
https://smile.amazon.com/Feeding-Masses-Planning-Events-Parties/dp/1462110711/
https://smile.amazon.com/Secrets-Caterers-Kitchen-Indispensable-Planning/dp/1557883521/  (I've since gotten this book ... most recipes serve 8)
https://smile.amazon.com/Cooking-Crowd-Recipes-Strategies-Entertaining/dp/1594860114/


Which also led me to Army cookbooks:


https://smile.amazon.com/G-I-Cook-Book-Authentic-Recipes/dp/147520650X
https://smile.amazon.com/Recipes-Cooking-Schools-annotated-study-ebook/dp/B009F7K7ES
https://smile.amazon.com/British-Army-Cook-Book-1914/dp/1445643421


Update: A friend mentioned a website with recipes for school cafeterias. There are some pre made sauces referenced, but it starts at 25 people and includes instructions for advance prep and just before service:  http://www.thelunchbox.org/recipes-menus/

Update 2: I've since learned that the preferred term seems to be 'quantity cooking', which helped me find:


http://recipesforacrowd.com/
https://www.foodhero.org/quantity-recipes (Oregon State University)
http://www.ellenskitchen.com/bigpots/