jbeech wrote:1. The ISO files will be 1080p30 . . . at the camera's bitrate, or re-encoded at some ATEM-bitrate?
The ISO has no idea what the camera's compression bitrate is. It only sees the uncompressed HDMI output coming from the camera. Also the files won't necessarily be 1080p30 since you can also set the ATEM for 1080p60 and other frame rates. The bitrate for the ISO files will be the max bitrate that the ATEM supports, which depends on the frame rate you've chosen but maxes out at 70Mbps for 1080p60 I believe.
2. Also, ATEM fed 720 will upscale to 1080. Presumably if fed 4K it'll also downscale to 1080, yes?
I don't think it will accept 4K inputs at all. Note that many 4K cameras only output (or can be set to output) 1080p on the HDMI port which is the only thing the ATEM cares about.
3. Stream from ATEM is 1080 at YouTube bitrates (user selectable).
Right. Also the recording of the program output will be at the same bitrate. That recording is the 5th video recording in addition to the four ISO recordings and will be identical to what was streamed to YouTube.
4. Otherwise, move the disk and edit the ISOs within Resolve for full bitrate projects.
Not just full bitrate but full resolution (could be 4K, 6K, 8K, or more) and also some cameras (BMD cameras in particular) might be recording in RAW which is a vastly superior format even when the resolution is the same and the bitrate is lower.
5. Camera originals save your ass if things go teats up with ATEM (especially if you get the project file).
Certainly
6. Redundant audio through mics on cameras (but a separate audio recorder is just prudent).
Not through the actual mics on the cameras but using the camera's microphone inputs to route the audio into the ATEM instead of connecting directly to the ATEM's microphone inputs. This can also be a good way to correct for video delay that the cameras introduce since the camera can correct for that automatically whereas you'd have to set up audio delay manually in the ATEM if you plug mics in directly.
7. Do you recall the file type of the ISOs?
The files are mp4 using the h.264 codec.
8. Wondering because ISOs are scaled to 1080 so who knows if they can/do encode, also?
I'm not sure I understand what you're saying there.
9. What SSDs will ATEM plug into with a cable that also go into MultiDock 10G at the Resolve computer?
I think you'd be better off just plugging USB-C SSDs (T5, Angelbird, Wise, etc.) directly into the computer and transferring the files to SSD drives in a MultiDock or whatever kind of storage you like. Otherwise you'd need another SSD dock connected to the ATEM to record to (which might work just fine but I don't see much point since you really want to make copies of your files ASAP anyway).