I would bet most "racers" will say you're stupid and give you all sorts of crap cause it's their way or no way... You know how the internet is these days... Personally it sounds to me though like you've nailed it spot on. Plates welded to the floor to spread the load out on the sheet metal and then tubes to those plates, 100% IMO the way to go in a rig like yours.
I'm imagining likely scenarios in my head and I would bet the most aggressive/hard impact you'd likely ever get into would be a endo on a departure or going over backwards on some crazy steep climb. Beyond that, I picture the occasional flop or even if it's a full barrel roll, it's not like you're going 80mph when it happens. I guess other worst case would be something like rolling down something like Hells Revenge in Moab where it would be multiple. Don't do that though! LOL, that's my advice!
Either way, in all of those scenarios I personally think what you have is going to be absolutely enough to save the occupants and do it's job quite well. All the bracing you have looks to be in the important areas to keep the roof structure from collapsing or folding over. It's not going to punch thru the floor.
If you tie into the chassis it's awesome and honestly I think it actually does help the performance of the vehicle but you instantly are going to get to start chasing cracks around your cab unless you get really serious with tieing the cab into the cage, IMO, too hardcore for what I picture you needing and not worth the headaches that would follow. Not to mention, once those cracks start, the creeks and noises that will come with them just driving around town will likely send you to the loony bin! LOL
Speaking of headaches, DO NOT SKIMP on roll bar padding!!!! You're not going to be wearing helmets like we do in our trucks. The cage may save your life but if you're a vegetable after the fact because you banged your head off a steel tube, did you really gain anything by having your rig caged? Doesn't matter if you have 10" of head space between your head and the tube and wear race harnesses, still pad it!!!! Trust me, things move, things stretch, stuff you will NEVER expect to happen can when it gets ugly and some padding will pay for itself if the worst ever happens! And don't use that pipe insulation from home depot, it sucks and hurts the wallet but buy some real race car roll bar padding material. Don't do all this work and stop short by a hundred bucks cause you cheaped out on a quality padding, especially since you won't have helmets. That's one of my biggest gripes!!! Mostly cause I know someone that ended up in a coma from a simple hit to the head on his roll bar tube, he didn't even actually wreck, just took an unexpected hard hit and bounced into it. Hasn't been the same since... BTW, you're caging it expecting a slow roll, if it happens on the street cause some idiot does something stupid, you don't want your head hitting that tubing when you're only wearing your factory seatbelt! Plan for the worst and honestly, that's probably it...
BTW, quick side story... Back in High school I caged our old CJ7. Kept the factory rollbar but ran 4 streamers forward to the top of the windshield and then a simple a-pillar type hoop. It was attached to about a 4" square plate welded to the floor. I went away to college, my pops flopped it over completely backwards on "El Hill" out in Coyote Canyon if I remember correctly. Of course the jeep was freshly painted, lol... But point being, from what people said it was a pretty solid impact straight onto the top of the cage after standing it up on the rear spare tire and going over backwards. Those plates to the floor didn't even bend the floor boards. Granted, lighter platform but still, it worked and did it's job just as expected. You still have the factory roof structure too to help out with. Although, if it's anythign like my '80's F150 cab I cut the roof off, I wouldn't rely on it much, especially compared to how insane the pillar support was on the '04 Tacoma we race when I cut the roof off it!
Oh, one more thing, and it's probably too late, those plates, if they're square, try to radius the corners so that there's no sharp piercing points to punch thru the sheet metal. If it's too late, I wouldn't stress at all but if not, it's a little extra touch I would do.
And lastly, if you ever wanted to tie into the chassis, you could probably slide a tube into the bottom side with a bottom plate and go to the chassis from it. You want that bottom plate to be a different size and shape then the top one though so that that two plates don't literally just act as scissors cutting thru your sheet metal. Point being, you can probably tie it in later down the road if you ever had reason to... I bet you never do though...