Advice on large-scale or multiple batches at once?

I'm a newb. I've done maybe 4 or 6 batches of partial mashes/kits.

In the interest of cutting down the time involved, I'd like to either (1) make multiple (different) batches at the same time or (2) do one large "megabatch".

Ideally, I'd like to get to the point where I can do maybe 100 or more litres at a time, and essentially set myself up for a good while in one shot. Not interested in going all-grain or buying a tonne of large scale equipment. Happy to do partials and I'd still be bottling it all. I'm envisioning basically buying extra fermenters and that's it.

Anyone got any advice on going this route? Does it cut down drastically on the time requirement by doing it all at once? Anything I should watch out for?

You have to boil all that wort, so if you want 100L at a time, you'll either need a huge pot and a burner to crank out enough BTUs (or joules, since we're going metric here) of heat to get it boiling for 60-90 minutes. After that, you could certainly split it into several fermenters and then pitch a full batch of yeast into each one.

If you have to boil each 5-10 gallons separately, you're not saving much time, although having one 12-hour brew day will help by cutting down on the prep and sanitation steps.

Something else to to think about... 15.5 gallon/50L kegs! This is mainly about space savings, but.... They make excellent fermentors, and if you did back-to-back brew days (maybe a day or two apart depending on what you are trying to accomplish that is) with the same recipe the first beer would act as a starter for your second and third batch (if you didn't have the 50L keg filled by the second batch that is). Not really a time savings but, this comes in really handy for lagers since you don't have to buy so much yeast and/or build such a large starter prior to the first boiled batch. Brew the remaining beers when the first is at high krausen, and the remaining "beer-to-be" will take off like a rocket. This of course doesn't help you with making different beers at one time.

Another thing you can do for more beer is the High Gravity brewing technique that a ton of the biggest breweries do. I like this because it gets me 15 gallons or more from a "normally" 12 gallon end of boil batch. Some brewers even stretch this into doubling their amount of finished product.

SilverZero is right though about the boiling. That is where you are going to save the most time. I know you said you didn't want to purchase bigger equipment, but kegs should be easy enough to find to cut the top off for a 50L boil kettle and propane burners/natural gas outdoor cookers are not a great deal of expense for freeing you up for such large batches at one time. I bet you could find a keg for free and a cooker for ~$70USD.

I don't see the point in not doing all grain. Partial mash takes the same amount of time and costs more. A cooler MLT is cheap. But then if you could do 10 gallon AG you could add enough extract to top off to 20 gallons in the fermenters.