In other words, you are considering a four way splitter.
You may as use a four way splitter
or one high current ESC that I just bought to have a look at and no opto leads if you keep inner & outer motors Y- leaded at present.
I'm up for that. It'll make room for your tools and cut down the wiring to a tenth.
Most suggestions will be Y-lead the left motors & the right motors as two pairs, as Rob47 suggested. It would give you the possibility of steering with the motors in addition to or instead of the rudder.
I further suggest plug two Y-leads into a tiny £2.50 v-tail mixer then into receiver.
If you don't get used to it in 5 minutes, take out the mixer to use the 4 way splitter or the esc I described in this and following post.
https://www.modelboatmayhem.co.uk/forum/index.php/topic,69839.msg758575.html#msg758575There are other options.
Fleet Systems, as standard, included a Tx switch that would combine twin throttle over on one stick. I think they also had an on-board hardware mixer available.
Some modern sets should allow you to wire in twin throttle & mix for single stick throttle then change to "four way splitting" mode.
Your description of the diagram is correct but I am doubting you see the diagram in the largest possible size.
Readers should be able to make out text and details within the three opto lead symbols after viewing the postimg page without even downloading the 800x 800 pixel image I uploaded.