Adding my two cents here since you are specifically worried about the 90Ds metering. Honestly, modern third-party flashes have nailed the Canon protocol so well that paying for the name brand is getting harder to justify unless you are shooting in heavy rain all day and need that insane weather sealing. If the Li-ion Godox units mentioned earlier are still a bit much for the budget, look at the Godox TT685II-C TTL Flash for Canon. Its basically the same internals as the V860 series but uses standard AA batteries instead of the proprietary pack. It has full E-TTL and HSS support, and I have found it pairs flawlessly with the 90D for indoor events. Just keep in mind that youll want some high-quality rechargeable batteries to keep recycle times snappy... the performance is almost identical otherwise. Also, since you mentioned harsh shadows, the real pro secret isnt just the flash itself, but getting it off the camera. I would highly suggest picking up a Godox XProII-C TTL Wireless Flash Trigger. This lets you place the flash on a shelf or a light stand to bounce light off a ceiling or wall from a much better angle than the hot shoe allows. It makes those community gatherings look like they were shot in a studio. For learning the technical side without getting overwhelmed, definitely check out the Strobist blog or the FlashHavoc website. They are basically the bibles for lighting. They explain things like the inverse square law and why bouncing light is your best friend. If you want a direct clone of the Canon flagship functionality, the Yongnuo YN600EX-RT II Wireless Flash for Canon is another solid choice that wont break the bank and works with Canons own radio system.

I've tested several units on the 90D and you really cant beat the Godox V860III-C TTL GN60 Li-ion Flash for the price. Honestly, the 1.5s recycle time is a game changer compared to AA-powered units. It integrates perfectly with the 90Ds E-TTL metering. With a 2600mAh battery providing 480 full-power pops, its technically more efficient than the pricier Canon alternatives for long events.

Building on the earlier suggestion, i've spent a fair amount of time comparing native lighting versus third-party alternatives over the years. In my experience, while the peace of mind with the brand-name stuff is nice, the price-to-performance ratio on the other side is just too good to ignore lately. I started out thinking i needed to stay in the official ecosystem for things like E-TTL to work right, but honestly, the third-party brands have the communication protocol down perfectly now. One small point i'd add is to look at the interface. Ive found that some of the newer, non-Canon units actually have much more intuitive screens and buttons, which is huge when youre trying to adjust settings in a hurry at a family gathering. You really dont lose much by going the more affordable route these days... just some weather sealing and maybe a bit of resale value.

If you're looking to fix those harsh shadows you mentioned, I'd definitely lean towards the Godox V1-C Round Head Flash for Canon. The round head makes a massive difference for indoor events because the light falloff is way more natural and soft than a standard fresnel head. It has full E-TTL and HSS which works flawlessly with the 90D metering. I've used it for a while now and honestly, the recycle time is super snappy because of that lithium battery. If that's still pushing the budget tho, the Godox TT685II-C TTL Speedlite for Canon is a solid alternative. It runs on AA batteries instead, so it is a bit more old school, but it hits all your technical requirements. Just grab some Panasonic Eneloop Pro AA Rechargeable Batteries to keep the recycle times decent. Godox is definitely the way to go over the overpriced Canon units nowadays, ngl.

@Reply #3 - good point! Late to the party here but I've been shooting with the 90D since it launched. While the Godox units are great, if you're really worried about long-term reliability and perfect E-TTL communication, I'd actually suggest looking for a deal on the Canon Speedlite 430EX III-RT. In my experience, third-party flashes can sometimes miss a sync when the camera is firing in high-speed bursts, but the native Canon stuff is rock solid. Here is why it might fit your needs:

• It's way smaller and lighter than the 600 series, so it wont make your 90D feel top-heavy during long family events.
• The interface is much more intuitive. No digging through weird menus when you're in a hurry... been there, done that, it's not fun.
• The HSS works seamlessly with the 90Ds shutter, so you dont get those weird dark bands in your outdoor portraits. If that is still a bit pricey tho, the Yongnuo YN600EX-RT II for Canon is a solid workhorse I've used for years. It's basically a clone of the flagship Canon flash for way less money. To fix those harsh shadows, just grab a Vello Universal Bounce Diffuser. Bouncing the light is the real secret sauce. Just point the flash head at a 45-degree angle toward a white ceiling and you'll get much softer results instantly.