The tier you actually need isn't the $20 one
Acuity's Starter plan is $20/month and gives you one calendar. Per Acuity's published tier list, selling memberships and packages starts at Standard ($34/month), and so do text-message reminders. For a class studio, memberships and class packs aren't an upsell — they're the revenue model. So the real comparison for a studio isn't $20 against $69, it's $34 or $61 against $69, with a calendar cap attached.
With StudioBook
$69/month with memberships, credit packs, single-class payments, and email automation all in the base price. There is no tier to move up to, and nothing about how you sell is gated.