That's a small number of wires for an ECU, in the industry over 250 was not uncommon that I was working with.
The joins are not uncommon either, I just hope the feed and GND are both big enough if there are only one of each. Though, the load calcs work out easily if you connect the right amount of highsides and lowsides correctly for outputs.

The sensor 5v is not really an issue distributing it, though as you are fully aware, takes a lot of planning when loom building!
Good luck