Allow users to select time of day to receive sms?

@suz_bru - I’m not sure if this falls in the “clunky” solution, but you add a “time-of-day” preference in your general Manage Preferences page.

Here’s my example:
 

For example, I have three radio buttons, one for morning, afternoon, and evenings.  If you want it to be more granular (like every hour of the day, you may want to use a Dropdown instead).

This becomes a “Custom Profile Property” for those subscribers who set this value, and then you can create Segments for each, and optionally a Segment for those that don’t have that property set. When you are ready to send your SMS campaigns, you can select those Segments of audience at the appropriate times (morning, afternoons, evenings).

Power Tip - You an use these profile settings in Flows too by conditionally branching by this property, and then use a Time Delay at a Specific Time of day. 

You may want to include a link to their Manage Preferences in subsequent emails, or if you can afford the space, in the SMS messages itself.  The subscribers’ manage preference links is a Klaviyo Template variable, here’s more information on it:

• Template Tags and Variable Syntax → Manage Preferences

Hope this helps!