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.