- There are a couple of things you said that caused me to squirm. I've probably wired a hundred or more houses, townhouses and condos in the last 40 years (I work, primarily, commercial jobs). I've done numerous renovations on homes too. I have never had a call back because of circuiting issues. It's usually the product failing.
When I design the electrical on a house, I always put the following 120v loads on their own circuits:
In the kitchen, all receptacles are 20A rated, on a 20A circuit with 12ga wire and only three receptacles to a circuit. All 240v loads are on their own dedicated circuit, in the kitchen and elsewhere.
General circuitry has no more than 10-12 devices on a circuit and are on 15A circuits with 14ga wire. A device can be a can light, receptacle, wall sconce or ceiling light. If there is a light fixture with numerous bulbs in it, I add the total wattage, divide by voltage, to compute amperage. Roughly, for every amp, I count the load as one device.
You should know the loads on a given circuit.
Always separate motors from general loads (lights and receptacles).
When loading breakers, I take the amperage of the breaker and multiply by .8 and determine that as my maximum load on that breaker. 15A breaker = max load of 12A. 20A breaker = max load of 16A. And so on.
I built my house with these rules in 1986 and the only time I've had a breaker trip is when a device short-circuits (the original cooktop just did that and it's being replaced). I have never had an overload trip. Usually, lights dim when there's a dip at the power company end but lately some original rotary dimmers have shown signs of failing with flickering lights they control. They are being replaced with electronic dimmers.
Service calls I've made regarding dimmed lights have been caused by:
- Landscapers nicked the underground feed to the house so every time it rained, some of the lights dimmed. Those lights were all on the same leg (side) of the panel as the nicked wire.
- Neutral wires were improperly paired. Wiring should be done with no more than two circuits (hot wires) per neutral. Neutral wires coming from the panel should never be wired together.
- The ground system was used for a neutral.
- Loose terminations.
- Imbalanced loads.
- Failing switching devices.
- Failing circuit breaker (rarely).
- Poorly seated circuit breaker.
- Power company issues.
There are probably other reasons but these are the ones that come to mind right now.
I'm telling you this because I can't look at your wiring and troubleshoot it. There are countless potential problems leading to your problem. But you stated you already had occasional dimming prior to the dimmer install. You also said your dishwasher and boiler are on the same circuit as some of your lighting. So the first thing I would do is take the dimmers back and install a different type. And I'd definitely separate the lighting and receptacles from all other loads, like the DW and boiler.
Since you plan on installing a new 200A panel, I would separate loads as described above and run the wiring back to the panel. When the new panel goes in, put them on their own breakers. It's not just load, it also makes servicing them easier.
Everyone has their own way of doing things. I'm sure some here will tell me mine is overkill, but everyone in my union local and surrounding locals all adhere to pretty much the same rules and things work just fine.
You can "get by" on less than I described. However you tackle this problem is up to you. But we have a saying here, "Short cuts lead to short circuits."