This button exists and it was made years ago(maybe even when the game released).
https://yadi.sk/i/8Q3au62kyghU0Q
Other options are low priority and not gameplay changing.
Countdown timer is not needed while there is ready check and battle starts right after the message that everybody is ready in chat.
Party/raid merging can only decrease time to invite players and not needed(3-5 seconds at all)
It doesn't matter who is tagged with you in easy raid, you just achieve it, because it is too easy. The better change is to make it possible to enter easy raid without waiting 10 minutes and letting afk people to pass this dungeon while 1 to 6 people are killing bosses