If you want to create cool shapes, then DSG3 is a must.
If you want to input/feed video into your system, then the Syntonie VU-003B is a must, it works with ESG3.
Regarding the differences between FKG3 (a single channel RGB variable soft keyer) and Keychain (3x single input hard keyers), the best thing is to watch the “3 Patches” for both at first, then read their product pages on here.
Here’s a page about the various Keying techniques used by certain LZX products but it’s technical, don’t look at it first please.
Here’s a link to the Doorway product page, it’s a keyer from the expedition series and can do both soft & hard keying. There’s a lot of info/detail on here but it’s not overly technical imo, hopefully it’ll help:
There’s also a link to the Doorway’s “3 patches” video at the end of it’s product page.
Ahhhh, there’s a Twitch Stream video from June 11th that went heavy into showing Keychain according to this post I made at the time.
There’s links popping up all the time on the LZX Discord server leading to the archive of the twitch streams that are found on a Google drive.
Ughh, I’ve just looked through the folders of the backedup twitch streams and the folders are numbered 1 to 26 but only a couple of the video files in the lower numbered folders contain the dates in the file’s title 
https://drive.google.com/drive/folders/16qsDG_v0OFGjhVoBNDxUiAmLb_q20qhf?usp=share_link