Potential wiring faults
During microwave operation, the microwave sound started to sound lower in pitch occasionally suggesting to me that the circuit is overloaded
That's not how overloads work. Your circuit is probably short enough that it has <1% voltage drop at max amperage. If you overload it 200% (30A) then you have 2% voltage drop. A triple overload (45A) would only be 3% drop. None of that would even be detectable at the appliance.
If the appliance is struggling, either
- a wiring fault is causing much worse voltage drop, meaning a connection is getting dangerously warm somewhere. Or
- a house-wide Lost Neutral is causing big voltage swings. Lost neutrals are insidious; they can go months before displaying clear enough symptoms to identify them. Or
- the appliance is dying.
This is plainly a Siemens/Murray panel, noting the un-plated copper bus bars which are a signature.
Due to differences in bus bar spec, breakers not made for this panel should not be in this panel. The panel labeling lists all allowed breaker types, and I'm fairly sure GE THQL and Square D HOM are not on that list lol.
All modern 1" wide breakers will seem to fit each other's panels but won't engage to the bus bar properly, which can cause arcing and fire damage down the road. So the Square D and GE breakers (I assume there's an Eaton in there under a label, just to complete the collection) need to go and be replaced with Murray MP (now Siemens QP). We're talking $5 a breaker in normal pricing, so this won't break the bank.
Bringing enough power
I usually use one or the other appliance, but not both. However, I might sometimes use both, but I imagine it will be rare, and so I don't know if it will be worth to wire up the circuit to account for using both, or using just one at a time.
You CAN'T use both. I'd explain why (both appliances are over 10 amps) but you have the technical chops to know why.
This is true of almost all kitchen heat appliances, as almost all are 12 amps (they run at absolute limit of 1500W @ 125V rating).
If you are "unable to control yourself" LOL, then you could install a 1-socket receptacle (simplex) so you'd be forced to unplug one to use the other. However, my view is that we're the humans! Electricity is our servant not the other way 'round, and *electrical should Serve Our Needs. Therefore if you want to run both at once, why be limited? Run 2 circuits. Easy peasy. Copper is cheap.
Well, not right now. If I owned plenty of 12/2 but had to buy 12/3, I would run two individual circuits and double-split the receptacle (or change to a 2-gang box). A MWBC on 12/3 is alright, but will require a two-pole GFCI breaker (not two singles).
Note that if 2 separate circuits feed the same receptacle yoke, they must be handle-tied, or a 2-pole breaker. This is the one case where you can handle-tie two single GFCI breakers.
Note, however that when measuring amps directly the ammeter gave a reading of 16.4 amps at the highest for quite a while, and then it went down to 14 amp reading otherwise.
Because it is sharing the circuit with 3 other outlets and 7 lights. The extra 2-4 amps are coming from other lights or plug-in loads on the circuit, which are doing their own things.
what AWG do I use?
You have no choice. Kitchen receptacles MUST be on 20A circuits.
So you must use 12 AWG copper (#10 if you have that).
#10 aluminum is also legal, but a lot of local inspectors will give you a fight on that.
what amp and type of breakers do I use, and where do I position them relative to each other?
what type of outlet do I use?
Kitchen counter receptacles require GFCI protection. That could be one 20A breaker and one GFCI repeptacle. Except that you have aspirations to run both appliances at once, which requires 2 circuits somehow. \
If you want to fit this into a 1-gang receptacle box, that means splitting the receptacle so each socket has a full 20A. (giving each a full 15A seems sensible, but is illegal per above). So these are our choices:
- A 2-pole (240V), 20A GFCI breaker, feeding a MWBC (Multi-Wire Branch Circuit) via 12/3 cable. To a single receptacle with the hot side "split" and fed from each hot wire.
- Two individual 20A GFCI breakers, with a handle-tie, placed adjacent since the handle-tie requires it. Dual 12/2 cable running to the receptacle. On the receptacle, both hot and neutral sides are "split". Each socket is powered from a separate cable.
If you are willing to blow out the box into a 2-gang box (or two 1-gang boxes), then you can use two GFCI receptacles, fed either from an MWBC or two independent circuits. In the latter case, the breakers can be anywhere. The MWBC breakers need to be handle-tied, so they will be adjacent. The cheapest way to get 2 handle-tied breakers is to use a 2-pole (240V) breaker.