Only if you view on a phone. I view social media on a PC and it is horizontal.

While I only have 2 ILCs, 1 bridge and 2 pocketables which all get used more or less, I can't explain the old 35mm film cameras I haven't even looked at in decades.

I still return to my Canon SX50 HS bridge camera occasionally. I took a great shot of the eclipse back in '17 with it. The zoom range is better than anything I have in my ILCs and is so much lighter than what would be required to get 1200mm (35 equiv) angle of view with them. It stands atop a tripod looking out at my back yard waiting for a good bird shot.

The 100-400 is an EF lens, designed for full frame or APS-C (crop), but the 18-300 is a crop body design. The image it casts does not fully cover a full frame sensor at all focal lengths. I'm not sure if a Canon full frame R series camera would switch modes on it. I would mind the significant loss of pixels on an RF full frame camera (or any modern mirrorless) when it goes into crop mode. That is why I went with the R7.

As I said in another post, I went from an 80D to the R7 and could not be happier. Using an EF→RF adapter I can use all my EF and EF-S lenses with no loss of pixels. I've added IBIS and awesome eye detection focusing as well as later a generation sensor.

Keep in mind that using a lens designed for crop bodies on a full frame sacrifices an inordinate number of pixels. You wind up using ~39% of full frame sensor pixels.

I went from an 80D to the R7 and am happy as can be with the choice. The R7 is an APS-C body with IBIS (In-Body-Image-Stabilization) and awesome focusing features like auto eye focus. I can use all my EF and EF-S lenses with no pixel loss with an EF->RF adapter.

I did just that. I use the EF-RF adapter and my EF lenses work fine on my R series Canon camera.

I shot the Aug 2017 eclipse with my SX50 HS bridge camera because I wanted the benefit of the full tele range. The zoom lens was essentially at full zoom which would be 1200mm (35 equiv). I still had to crop the resulting image slightly, so 200mm on a full frame isn't much.

I used internal exposure but the EC was at -3 which worked out well since the sky was very dark at full eclipse.

You won't have much time, in fact hardly any at all. Have the rig set so you can frame and focus while standing or sitting comfortably. If you are using a sun filter you will need to remove it for the full eclipse, or have one camera filtered and another not filtered.

That darned Murphy!

Flash sync is the fastest shutter speed that has a moment where the entire sensor is being exposed. Any faster and the 2nd curtain starts to close before the 1st has traveled across the full sensor. It isn't a setting, it is a physical limitation.
Shutter speed is the setting of the exposure.

Check! It has the same wide open aperture available for all zoom settings.

You've heard of TMI (Too Much Information)? This is TLI (Too Little Information). Much depends on the 9 year old's personality. Without knowing that, I can't make a recommendation. If the child is high energy something like the tough TG6. If quiet and curious a bridge camera might be best. Otherwise...

Camera prices are regulated, they can't be discounted by authorized dealers. However, memory cards are not so to add value to an offering they add it free with a camera.

The only current Canon in that series is the G7x Mark iii. I went with the Panasonic DMC-ZS100 which has an EVF that the Canon does not and it costs less.