Aside from the alien breaker problem ThreePhaseEel mentioned, and the neutral wire chew-down JACK mentioned, I see a couple more problems.
Too many breakers
You have 7 throws to turn everything off, as 'nobody' mentioned. Ed Beal has a visionary solution of putting "handle-ties" on the top 4 breakers... but unfortunately that won't work because of this line in the literature:
INSTALL NO MORE THAN SIX CIRCUIT BREAKERS.
You have seven. So your only option there is to buy two "2-pole" 20A breakers (QO220) and replace the top 4 breakers with those. It means those circuits will have common trip now, so if one overloads it knocks out the other. If the pairing of the circuits makes that inconvenient (table saw, and shop lighting!) then switch one of the hots with the other breaker.
You can do that because you have extra length on wires (which you referred to as "messy"). No, messy is having a bunch of wire nuts because you had to extend wires.
I say do both of them so you now have five, so you don't have to think about the problem again when you decide to fill that last space.
(and no, not six quadplexes (18 hand throws) -- the literature forbids tandems and quads, so don't go thinking you can start double-stuffing this panel when it gets full. This type of panel is not for that!)
I'm also concerned about overload here.
The main reason we care about obsolete and dangerous "Rule of Six" panels is that it's all too easy to overload them. When they were legal, the argument was "there's no need for a master circuit breaker; the home is protected by the Load Calculation that determines the home is unable to draw too much, and any overloads will be occasional and transient".
(Of course, this was a conceit; the real reason for Rule of Six was that >60A breakers were prohibitively expensive at the time. That is no longer the case.) *The point is, this scheme depended utterly on the homeowner doing another Load Calculation every time they added a load. As you may guess, that never happens.
So what's going on in this panel? You have a 200A breaker feeding a sub panel, and if that were the only breaker in this Rule of Six area, that would eliminate the safety problem! However, we have more:
- 30A of load on both poles from a 30A/240V breaker
- 20A of load on both poles from two 120V breakers on upper left
- 20A of load on both poles from two 120V breakers on upper right
- 20A of load on one pole from the remaining 120V/20A breaker.
So I count 270A and 290A of breaker trip on the two poles. Now I'm not saying the 200A breaker should be changed for 100A (giving 170A and 190A) -- after all, some oversubscription is tolerable, because it's statistically unlikely all those breakers will be maxed out at once. But certainly, grafting them on top of 200A with no derating at all is too much.
So since you need to replace the large breaker anyway because of the alien idiocy, I would suggest replacing it with a 150A breaker. That will leave 50A of headroom for those various breakers to share.
How I'd do it, though.
Another way to do this, since most of the hillbilly-installed Romex must be replaced anyway, is to install a new 200A main-lug sub-panel right here. Reroute all those circuits (the five 20A and the 30A) so they are in the new sub-panel. At that point, the 200A breaker will be the ONLY breaker in this "Rule of Six" panel, eliminating the safety problems of Rule of Six panels altogether.
There are two ways to get power to this new sub-panel.
First, you could start at the interior panel, stick a 60-90A breaker there, and use 2-2-2-4 MH feeder to come back to the new sub panel. A bit of a "round trip" but there it is. OR...
Second, you could get "double lugs" for the new sub-panel. Use those to daisy-chain from the meter-Rule-of-Six panel... to the new sub panel... to the original interior panel. Both panels are rated for 200A, and they're both protected by a (the same) 200A breaker, so they are fine that way.
Third, oh, this is the best one, for cost and versatility!!! -- You know that "Alien" 200A Siemens breaker that we have to get rid of? Make sure the new sub panel is a Siemens brand. It's not alien there! That will save us the price of two double-lugs. And one more thing: wire this from the meter-main to the Siemens breaker, so the Siemens will now be "back-fed". Then wire from the subpanel's main lugs to the interior panel. This is "backwards" obviously, but this way, we can simply install a generator interlock between the fat Siemens breaker and the generator breaker, and switch this sub panel as well as the house's interior panel. "Better and better!"
"But wait, under this "Best" option we will have a brand-new 200A Square D breaker One Foot Away from a 200A Siemens breaker. Why do we need 2 breakers? Can't we just forget that expensive and scarce Square D breaker and use a sub-feed lug kit and run totally un-fused power One Foot to the new sub panel?" Maybe... that would make sense. That is a question I would ask your AHJ. (local code inspector). It is their final call. My advice there would be to use a metal conduit nipple between the two panels, as that will carry safety ground and make it look pro. PVC between panels is the mark of the amateur.