The problem is that most boats current wiring is likely long modified from the original schematics
If all you are doing is upgrading the fixtures, can't you just use the current wiring? For example you mentioned lighting. If you are changing to LEDs, the current wiring would be of ample amperage.

I find on smaller boats is more practical to have multiple panels. It's not difficult , unles you deside
complete re wiring. Than you need schematic with all required specifications. Current ratings of individual
instruments, current ratings switches / or relays if switch panels are under rated/, fuses.
Most importantly tinted wire correct gauge, not only to carry required current but inrush current too.

In case you just add on, process is simple, correct gauge tinted wire, fuse, switch and terminals on both ends
to fit your current wiring.

dism,
There are several things to consider. I had a commercial switchboard and all my lighting is LED. We found that the lights on the switch board were not LED and were in fact my biggest consumer of power. I have changed them for simple switches, but I have a LED light for night time viewing of the switchboard.
With navigation lights you need to consider the 3 situations; at anchor; steaming and sailing. For steaming I have lights at deck level and half way up mast, anchor and sailing on the mast head.
Then you need to consider your radio/AIS/plotter/ depth sounder setup. For each of these to read each other they need a common earth and an earth between all of them [this is so they can see the gaps between the NEMEA data. Very important or they cannot work together].
I ran separate wires port and starboard to pick up the inside lighting and separate wires to each instrument. In fact I ran a dedicated positive from the house battery to all my radio/plotter/AIS via a switch for each with a fuse as I found that power spikes when starting the inboard cause all these instruments to switch off to protect themselves, very frustrating if you had to start the engine in a tight situation and I lost all the days tracks etc as well!
I have 2 house batteries and a starter battery which are all charged by a 100W solo panel via regulators. I have no frig [yet] but do run my autohelm up to 12 hours at a time and I have never suffered from lack of power with that set up.