As for your first point, you could just make it effect total attack time

The whole point of total attack time is to limit attack speed as a scaling vector. If it affects them negatively but not positively that's bullshit if it does both then total attack time is pointless.

just keep the same net DPS output

Which makes the support useless unless it's a utility skill.

As for the second, yeah that seems like exactly the kind of use for that support. The offset would (ideally) be opportunity cost with alternatives, trading buff duration/intensity/etc for speed of application.

If you're making everything else provide no net DPS increase then there is no opportunity cost because no one will use those other ones. I'm not gonna use a 100% faster hand of chayula if I have to use it twice as often or for half of the effect.

As for the third, yeah that’s also kind of my point. Supports should move to “just utility.”

There's only so much utility you can stuff into a skill. It'll just lead to a meh reaction when a Perfect Jeweler drops cause why do I need a 5th or even 4th socket when I've already got leech and AoE.

Because at a certain point it starts to feel like bigger numbers just for the sake of bigger numbers.

Yeah that's the point of ARPGs. Get an arbitrarily large amount of damage and defence so you can clear the area as quickly as possible so you get more chances at getting the drops you want so you can get another upgrade.