Indeed /castsequence can only have a single [options] block.

You can try mouseover macros like /cast [@mouseover,help,nodead] Arcane Intellect if you ensure that the unitframes are in the same place for every toon (e.g. sort alphabetic) then it can be cast from the active toon.

Some people use auto-buff addon, but I don't have any specific suggestion.

If you are using ISBoxer there is the possibility to use the {FTL} macro extension to make a sort of 'buff me' macro, similar to the standard 'assist me' and 'follow me' macros. But then you'd have to switch to each toon, so that's 10 button presses for a 5-man group.

Buffing is not trivial in classic. You can reduce it to 5 main binds. I have O=fort/motw ctrl-O=int/spi shift-O=blessings Y=thorns/inner/mage/demon/trueshot and shift-Y=sharpening/poisons. Then of course there are the less-used ones like fear ward, shadow protection, dampen magic, underwater breathing, detect invis, etc. Plus you need a way to monitor all of these in case they fall off.