I also dislike the any-% in-element deck per match. Trials are about to master your element of choice, including making use of its strengths and making up for its weaknesses. Using off-elements need proper planning to be successful so you don't get screwed. When you can field any deck at any one time in the match, this planning becomes mostly obsolete. Maybe something like the Tinker role from a few wars before could work, but not sure about this (for reference: With Tinker, your deck's mark had to be your element's mark, and all your Pendulums counted as in-element). With that, you could still technically field monos, but with a drawback, but it will make some duos or even trios more viable by giving them the Pends with the quanta you want to generate first.
I do not see 40 % as that big of a problem as your own element will still make up most of your decks, so trying that out seems like a good idea.
A proper reason for bow bans would also be great. It is totally fine to restrict its usage to maybe 2 times, but leaving them out completely will restrict an already tight meta even more.
I might have missed the rule, but are these 8 off-elements for all your matches (probably not counting the I-hate-Brawl tasks?) or for each match?