The problem with load calcs in a standard resi application is they dont properly account for diversity and will have him way oversize his panel.
Sidebar - true story time - the manufacturer I rep for currently has a new tech product they are bringing to industry that Ive been involved with a beta/pilot with for ~15 months as a technical consultant and local SME. This product is aimed at homeowners. And it does a number of things but the point relevant to this conversation is its load and usage monitoring capabilities. We currently have 200 of these devices deployed in NC and SC across a strategic range of houses. Over 70% of the homes are finding that their actual peak instantaneous consumption is below 50A. These are on services ranging from a standard 200A resi service, to a 400A heavy resi to in 3 cases a 3x200A resi multi service.
Now the nature of our business and the concept of design days meas we have to over size for worst case scenario. All of us in the loop get it and understand why this is. But someone who doesnt have a lot of electrical experience this doesnt make any sense to them.
Hence you get the OPs concern that he has 1020A connected to a 200A main and it seems like the main will "fail" because it cant handle this.
So for the point of education I will offer this to the OP.
Your utility (Most likely Duke) brings a service into your house that can carry in practicality any load you put on it. Its a niagra falls supply of water.
At the top of you panel (top theoretically speaking in actual practice of your pic its at the bottom) is the main breaker that corrals all that water and only allows 200A to flow through. You can back up 10Gigawatts of water and that 200A funnel will only allow 200A through. That is the point of service entrance. Then that breaker puts up to a max of 200A onto a copper bus in the back of the panel and it has -in this case - 42 taps that can each take some of that water. In theory you could have 42 200A taps there (not really for a couple reasons but just play along) but if you had 42 200A feeds each of those feeds would have to be rated for 200A. This means the water pipe would have to be sized to flow 200A. In this case that water pipe is wire size. Instead they choke these down because they don't need 200A at each faucet they only need 20A. So a 20 breaker can hold back the full 200A that is being limited by the main but only allow 20A to flow through its gate. The 200A is standing at the top (bottom in this case) monitoring all those little taps and measuring them all and if they all use more than available it shuts off the tap. If one individual circuit is grossly offending its portion then its individual breaker will trip and take it out of the mix.
That's why you have over 1,000 amps of breaker fed from a 200A breaker
One final point of thought that will probably help OP sleep better.
There are 5,040 hours in a 30 day month.
A KWH (how you are billed for electricity) is a kilowatt hour. That is 1 kilowatt of power continuous for 1 hour.. (for example 2 kw used for 30 minutes is 1kwh or .5 kw used to 2 hours is 1 kwh)...
1 kw at 240V single phase is roughly 5 amps(4.16 for the nerds).
If your house used 5 amps of 240V continuously 24/7 that would be 5,040 kwh per month.
Duke varies a little by location but NC's blended average is 11.5 cents per kwh.
Meaning if you used 5A continuously for a month your power bill would be around $580 per month. You can look at this and quickly realize your average usage is probably much, much less than 5A TOTAL. Across your entire house.
That helps to illustrate why you can have a 200A panel and add a 75A sub panel to it and not over load it.
That's the background on why.
Now this is not professional advice in any way. This is just a random dude typing on the internet who probably doesnt know what he is talking about. Take it at face value..
That said if I was doing this I would pick up:
about 5 foot of 3-3-3-5 SE Copper cable. My local home depot caries it and its about $6/ft.
(You can buy aluminum wire, most people would, I dont like aluminum wire if I an avoid it for sub panels and short runs. Since we only need 5-6' Id pay the premium.But if money is tight you could save about $5 using aluminum wire but it would need to be larger.)
Then I'd pick up a Square D HOM2100C breaker. That should run you about $35.
Next Id pick up a sub panel. For this case a HOM2040L125PGC will work fine. That will run you $50 (That's a main lug only - you don't need a main breaker in the sub panel. You have a main in your current panel and the new breaker above feeding this panel)
You will need some breakers for your new panel sized to your new equipment, but this panel will give you the space to relocate a couple circuits from your existing panel over to this new panel. You will need a couple of connectors at each panel for the SER cable to come in and out of. But other than that you will be in business.
Also - of course you will hire a licensed electrician to do all this. Any one worth his salt will know this, but shockingly lots of folks carry license and dont know how to change a light build these days. Just glance over his shoulder, or ask innocently to be sure he doesnt bond the neutral and ground in the sub panel.
