Seconding the recommendation above for sure!! Honestly, the Godox V860III-C Speedlite for Canon is a total powerhouse and fits the 90D like a glove. I've been using Godox gear for years and the value-to-performance ratio is just insane compared to the official Canon Speedlites that cost twice as much. Since your budget is up to $300, I'd actually suggest looking at the Godox V1-C Flash for Canon too. It's basically the V860III's sibling but with a round head. Why care? - The light fall-off is way smoother/circular, which is literally a game changer for portrait work when you're bouncing off a ceiling.
- It has a magnetic rim so you can snap on the Godox AK-R1 Accessory Kit for diffusers and gels in like two seconds. Both of these use that beefy Li-ion battery, so you wont be fumbling with AAs mid-shoot. The recycle time is insanely fast—approx 1.5s at full power—so it keeps up with the 90D's fast shutter reallyyy well. Plus, both have the built-in 2.4G wireless system. If you ever wanna get the flash off-camera, just grab a Godox XPro-C TTL Wireless Flash Trigger and you're golden. Full E-TTL and HSS work perfectly on my 90D with these—metering is super accurate iirc. The menu integration is also great; you can adjust flash power directly through the camera's "External Flash Settings" menu without even touching the flash itself. Basically a no-brainer at this price point. gl!

For your situation, i'd suggest the Godox V860III-C for Canon. The Li-ion battery is realy a game changer for that 1.5s recycle time, and it works great with 90D menus... TL;DR: Grab the Godox V860III-C for Canon for fast HSS and E-TTL on a budget, you know?

I've used tons of setups with my 90D!! r u looking for lithium-ion for *sub-1.5s* recycle times, or r u cool with AAs? also, whats ur burst rate??

Seconded!

Been thinking about this for a bit and honestly, my gear bag is kinda like a graveyard of old flashes I've tried over the years. When I first got into portrait work, I was a total brand snob. I thought if it didn't have the Canon logo, it would probably fry my 90Ds hot shoe or something crazy. I spent way too much on their high-end gear only to have it overheat during a summer shoot, which was super embarrassing. In my experience tho, things have changed a lot.

• The official brand speedlites are reliable, but the price tag is honestly hard to justify anymore.
• I've moved almost entirely over to Godox for my professional work and never looked back.
• The lithium battery systems they use now are a total game changer compared to the old AA battery days. Just get any of the newer lithium-ion units from Godox. You honestly cant go wrong with their ecosystem. It talks to the 90D menus perfectly and the recycle times mean I'm never waiting around while my subject gets bored. It's just so much more efficient than what we had ten years ago, and way easier on the wallet too.