Sure sounds like you may have a couple of unrelated problems. Usually, a nuisance tripping GFI is a sign of a worn or aged GFI device. This is assuming there are no down line devices or lights that are leaking small amounts of current to earth ground. This would be uncommon for a light or simple switch, more likely with a motor load on start up or shut down. The more important and potentially dangerous problem is the whole house lighting flickering when the fan and shower light is on. If you have an accurate digital VOM, you can look at the voltage drop at the breaker that controls the bathroom. If you see variations of much more than 5 to 10 volts then isolate which device is causing the fluctuation. (fan or bath lite) by turning them on individually and test again. A older fan is a common source of your problem as moisture and humidity may have invaded the motor and causing larger than average voltage drop to ground. If this fan is in fact the culprit, the fan may be heating up more than normal as well and could become a hazard. If you do not have a VOM or know how to test voltages inside your circuit breaker panel, I'd advise you to seek a licensed electrician soon to correct the situation.
Bathrooms
Bathroom fixed loads like lighting and fan can be served by any convenient circuit.
With the bathroom receptacles, you have two choices.
- Many things, one bathroom: The circuit serving receptacles in one bathroom can only serve other loads (fixed loads, lights, fan) in that same bathroom. or...
- Many bathrooms, only receptacles: A circuit may serve receptacles in any number of bathrooms, but it can serve only receptacles and only in bathrooms.
You can go either way.
Switched receptacles
When having split receptacles (one unswitched, one switched)... the usual way that will be most understandable by "the next guy", and keep box fill sane, is to do switched receptacles is off the same circuit that powers the unswitched receptacles. So in that case, you connect 12/3 between the two split receptacles, and from one split receptacle to the switch. Remember, you need to carry /3 to a switch loop because you must bring neutral in a switch loop. You cannot use neutral from the other circuit!!!
Because you will have two different circuits in the same box, you must be very careful not to mix them. You might even consider putting this switch in a different junction box, to remove all risk of confusion.
Having made the /3 connections between the two receptacles and the switch, you now can supply power in any of these locations - or really, anywhere else in the circuit. Since I would be worried quite a lot about box fill in switch #1, I would consider feeding power into the receptacle circuit at the near receptacle.
Off the far receptacle (or anywhere in the circuit), you then continue with 12/2 cable carrying always-hot and neutral to other outlets.
The second way to do it is to put these switched sockets on the lighting circuit. In that case, the switch box would be all one circuit and that would ease box fill there. But now you are running two complete circuits to the switched/split receptacles, although you can use /2 cable. You will need to break off both the hot and neutral tabs and take care to keep separation of circuits. Which will make box fill worse at the receptacles.
Closet lamp #4
Given the proximity of closet lamp #4 to a receptacle location, consider powering closet lamp #4 off the receptacle circuit instead of the lighting circuit. This will need #12 wire because ALL wiring in a 20A circuit must be #12, no exceptions. It is totally legal to do this, it's just not recommended to put lighting on receptacle circuits generally, because if you overload a socket, the breaker trip will plunge you into darkness. That's not a big issue on a single closet.
By the way, no receptacles allowed in closets, and lamps can't be where clothing can touch them.
Box fill
Switches 1 and 3 are going to have a rather significant box-fill problem. I believe we discussed this six months ago on the three-fan circuit. Be careful selecting your boxes so they are large enough, I'm fond of 4-11/16 square steel boxes and either 1-gang or 2-gang mud rings.
Best Answer
You can share GFCI receptacles. You might have to.
OK. Here's a science fact you do not know, and neither do the people who wrote that instruction.
A GFCI is not a receptacle. It is a system of protection that can protect any number of outlets. In fact they make GFCIs that are not receptacles - GFCI only, or GFCI+breaker, or GFCI+switch.
Anyway, every GFCI device can protect other equipment that is wired downline of it... it has a set of terminals called "Load" which are for exactly that purpose and no other purpose.
Now, obviously, a GFCI receptacle protects its own sockets but it still has those "Load" terminals that you're not supposed to use for anything else.
So GFCIs have 4 screws - 2 Line and 2 Load. Unfortunately, plain old receptacles have 4 screws. So when novices upgrade receptacles to GFCI, they just go "la la la, 4 screws = 4 screws done!". So they are accidentally putting the downline on the "Load" terminals, giving it protection they don't even realize.
Those instructions are written for novices, and so they told you it needs to be a GFCI receptacle. It does not: a plain receptacle fed from the "Load" terminals of an existing GFCI receptacle is perfectly fine for that requirement.
So you do not need to relocate the vanity GFCI for this purpose alone: you can simply feed off its "Load" terminals to grant GFCI protection to any other plain outlets in this same bathroom.
In fact, you may be required to: Code does not like GFCI receptacle in inaccessible locations, because it's too difficult to do the monthly required test.
However, the bidet requires a dedicated circuit
It's right there in the instructions, which you must follow (NEC 110.3(B)).
So that means your only wiring option is 1 circuit dedicated to the bidet, and the other circuit used for all other loads.
Bathroom receptacle circuits must be 20A
Keep in mind if this is NEW wiring, bathroom receptacle circuits must be 20A. All of them. You cannot use a 15A MWBC (shared neutral) as a substitute (though a 20A MWBC would certainly do the trick).
If you cannot meet the "accessible GFCI" requirement, you can fit a GFCI deadfront (which is a device that is GFCI only) at an accessible location.
Really, the correct way to wire an outlet for a dedicated circuit is to use a 1-socket receptacle. Those are not made in GFCI (except for GFCI+outlet+switch combo device) and so again, the deadfront might be the way to go.