Wiring for an emergency generator interlock
Edit because I can now see your pictures:
Best bet if it's a convertible panel
Call around to electrical supplies until you find an Eaton dealer.  Give them your panel model number and ask them if it's convertible to main-breaker and if Eaton has a factory generator interlock for it.   I have a feeling it's convertible because some features in the top area (like those funny pillars) don't make any sense otherwise.
Wow, that was super easy.
If it's NOT convertible, note the sticker &quot;Use in Box G1 size (indoor) or G1R size (outdoor) panel&quot;. That refers to the steel enclosure (the thing 14&quot; wide by 30&quot; or so tall).  Search for a panel that uses the same exact box as yours that is convertible - example, a BR4040N200R panel.   Pull all the breakers and tuck them back, and remove the two fat hot wires.  Then remove the screw now hiding under the bottom breaker and the screw between the pillars up top. There may be 1 or 2 more screws, and voilà, the bus assembly pops right out.  Do the same to the breaker panel you just acquired, and swap the bus assemblies. Also swap lids. You have replaced your entire panel with astonishingly little work, because you picked exactly the right donor panel.   Now you have a convertible panel; proceed as above.
The next best thing: Backfeed a breaker
Another option in this panel is to fit a 125A breaker in the top position of your panel, and backfeed that.  (Sadly BR does not make a 200A plug-on breaker, and you can't use any other brand. So you'll lose a lot of usable ampacity.)  You would remove the feed wires from the main lugs, and reroute to the backfed breaker.  Then use a backfeed style interlock (BRPMIKBR) to interlock that with your generator breaker.   Most likely these (4/0?) Al wires won't fit on the breaker so you'd need to change to 2/0 Cu.
If you can't make your panel convertible, use a transfer switch
That's your idea, and yeah, sure, I don't see any problem with that.  You could use a main-lug panel and the kind of interlock kit where two &quot;normal&quot; breakers backfeed and are interlocked.   One breaker is &quot;Utility&quot; and the other is &quot;generator&quot;.  Both backfeed the bus of that panel.   Then off the main lugs of that panel, go onward to your large existing panel.
If you feel weird backfeeding and sending power &quot;upward&quot; through the main lugs, then mount the panel upside down so the main lugs exit the bottom. Then just pretend they're feed-thru lugs lol.
Edit: One big snag with this is your 200A service.  That's an awful lot to backfeed into regular breakers simply because 200A regular breakers are scarce. What you need is a panel type that both a) supports 200A branch circuit breakers, and b) has an appropriate interlock.   That's a scarce combination - for instance Square D's lowbrow HOMeline is deprived of many features, but it actually does have a 200A branch circuit breaker (HOM2200BB)... unfortunately not a factory interlock, so you'd have to go to the aftermarket.
Probably a better bet to go with any of the numerous of 200A main-breaker panels out there with 4-8 breaker spaces and feed-through lugs, such as the BR816N200RF.  You could stick the generator breaker in one of the normal spaces and use a standard factory interlock, then feed the big panel off the &quot;feed-through lugs&quot;.  The only problem is, the boxes are gigantic (for cable bending space reasons).
Either way, the above boxes could actually stand in as a substitute for your current 200A main breaker/disconnect (that's kinda what they're for)... if you wanted to get into pulling the meter / pulling permits / inspection to get the power turned back on etc.
But if you're going with a gigantic panel anyway...
... just have a second main panel and call it good. Another 200A main breaker, this time panel right next to your existing panel, with interlock.  It's fed with 200A wire either from your main breaker outside, or the panel next to it via double-lugs or &quot;tee&quot; Polaris connectors.  Serving 200A wire to both panels is fine.  This lets you take a clean-slate approach without having to move all your circuits. Simply move the circuits that you want on generator.
Or if you're moving circuits anyway... go with a smaller panel.
This lets you use a more compact subpanel such as a BRP20L125, with two backfed breakers interlocked. One backfed breaker comes from generator, the other from the large panel where it's fed from a breaker there.   Then you move into this subpanel any circuits you want on generator.