Starter healer... WHM?

[Rilifane]

snip

Sacred Soil is actually 600 potency since they changed ground target aoes to give an instant tick upon placing them. Which makes SS the strongest heal ingame because its 10% mitigation puts it ahead in pretty much every scenario of even Earthly Star for aoe heal and makes it comparable with Excog for ST in many cases aswell.

Too bad there is far more whm/ast combinations in savage and hard content vs sch to me will be inferior to whm ast, you gotta be wasting all these precious time to enhance your heals to make it better than whm/ast which its still dont win

AST/ WHM is so popular because WHM is popular, not because it's stronger.
And a class being popular doesn't automatically mean it's strong, it just means its popular for whatever reason.
In case of WHM it's mostly because some common misconceptions that keep getting spread by a community of people with half-knowledge at best.
WHM is lauded as the straightforward healing powerhouse when it's in fact the opposite.

WHM is:

• very immobile, by far the most immobile class in the entire game
• it's very limited mobility is tied to using instants meant for healing and a dps loss outside edge cases
• very few weaving slots
• it's limited weaving slots mostly tied to using instants meant for healing and dps loss outside edge cases
• by far the lowest dps neutral aoe and ST healing potency of all 3 healers
• much lower mitigation compared to SCH
• suffering from conflicting design of skills

Meaning WHM requires a highly coordinated party with well-planned mitigation, good positioning from everyone and a co heal that can carry the majority of healing to be effective - and even with all that it's still behind AST/ SCH by several 1000 combined healer dps across all gameplay levels and bosses.
The only unique strengths WHM has are Bene and Cure III but I can think of exactly one mechanic on one boss when it was strong: Terminal Relativity on e12s P2. The very last mechanic of the last boss.

SCH has ghosting issues and several skills make no sense, like Fey Illumination only increasing healing magic and not affecting the fairy or Seraph locking you out of other fairy skills instead of being an all-around upgrade and they constantly need to reposition their fairy after it resetted from Seraph.
Nonetheless, it's overall far stronger than WHM and it doesn't even require the party, but espcecially your co heal, to adjust around your countless weaknesses.

SCH's dps neutral healing potency is quite strong and it doesn't suffer the same issue as WHM when your healing is more or less dps neutral for a short time and then completely locks you out of dpsing once those few options are exhausted.
It's much more gradual with SCH, meaning they don't suffer noticable dps loss unless the party is really, really hilariously bad.

There's also the benefit of AST being able to go diurnal even in early prog with a SCH, meaning it can use its overall stronger toolkit instead of using the overall weaker heals of Noct just so they can have unlimited shields on demand in addition to Neutral/ Seraph.
Between Seraph and Neutral, you already have plenty of shields and in the few cases that's not enough, Succor may be weaker than Noct Asp Helios but will do the job just fine.

I know it's a bit off topic but I always think the best starting healer is SCH.

You have the fairy there to constantly be healing if you're slacking a bit.

You can switch to SMN to do the MSQ faster.

By now I usually recommend SCH to people who want to try healer aswell for those exact reasons.
MSQing as a healer is the sloggiest slog of all slogs and the auto heal makes it so much easier to incorporate dps from the start with more room for error instead of falling for the classic "I'll just heal everyone first until I'm suuuper confident nothing can possibly go wrong and then use one dps skill".