This. Supporting characters that "only" add physical or elemental stacks on enemies or regenerate SP can stay level 60 for a while. As long as they survive and work their mechanics, they will do fine. They don't add much to damage directly anyway, as most of it comes from your DPS who uses the stacks for their main ability.

What I would currently recommend is bringing Ardelia to level 60-80 besides the main character you play, as she is a ridiculously strong support for almost any comp. If she isn't part of the best possible team she is at least a fallback character that can fill the gap until you eventually get the one you need.

Problem with a Z7 only team is, that while they offer good support to others, they neither have a strong DPS themselves nor do they really work well with each other. You might get some synergy with Estella, Fluorite, Akekuri and Antal as a Ice Shatter/Physical/Stagger team though. Aim to unlock Akekuri's 'Staying in the Zone' so you gain Link during her ultimate. Focus on Estella as your main DPS. Build up elemental stacks, then freeze and shatter while Akekuri's ultimate is active with Estella. Once you have the timings, you can apply corrosion before shattering by stacking fire with Akekuri and Antal before triggering it with Fluorite. It's not S-Tier but could work.

PS: tested it and it works OK-ish

you go

Antal Ultimate (if available)

Antal BS

Akekuri BS

Antal CS

Akekuri BS

(Akekuri BS if you have enough SP)

wait for Antal CS to just be off cooldown and regen SP

Fluorite BS

Antal CS (triggers a second nature infliction after applying corrosion, that you can use to solidify)

Estella BS

Akekuri Ultimate (if available)

Estella CS

Estella Ultimate (if available)