Hmmm... with that many devices, some of them time sensitive, (phone, etc) might it be bloat related issues? Have you tried testing for bufferbloat?
As has been said, 200mbit should be OK, maybe not stellar. But, if bloat is the issue here, and I'd guess so with the traffic load you describe... you WANT SQM, and then your C7 is only good for 100-120mbit. So a problem, or a lot of speed to give up.
You could set up Cake, throttle it back to 95-100mbit and see how that works. The more advanced settings might help your traffic mix more, as well. See the various SQM pages.
Another trick I learned, when I was running just a C7 and had a 300/30 connection, was to only SQM the upload/ingress. Set it to 0 and 30 for the rate speeds. Then you get full line speed on the DL, it just wont manage flows on it. The C7 easily handles SQM'ing 30mbit, and I saw much of the total bloat improvement of running at 100/30. This of course, wont help if you're symmetrical and have 200/200.
Other thing to do, is migrate to a more powerful box, or split system like slh mentioned above. The C7's make decent AP's. I do this now, and its great. A little mini PC or a RPi4, and you're not burning much power.
You could try one of the simpler queue disciplines, but I dont think that will get you to 200mbit. Beware of the C7 getting to higher speeds with SQM than saturates the CPU. Cake at least, will just pass w/o processing if it runs out of CPU, and it looks like its doing 250, while it isnt doing much SQM anymore. Check CPU idle time in top, or watch carefully in the bloat test. A C7 w cake maxes out about 140 on the wire, and about 100-115 over wifi, as I recall.