How to send a thank to a recipient of a soft credit
I've been writing extensions these past few weeks to address this very scenario, but I haven't finished them completely and haven't advertised their existence yet - in part because of improvements to CiviCRM that aren't in the released version yet.

Ready Now

My Soft Credit Tokens extension adds some name tokens for the soft creditee.  Note that I've only tested this with CiviRules, but should work in the scenario you mention.

There's also Send to Soft Creditee which creates a CiviRules action of the same name, but I don't recommend its use as more than a stopgap for more than a couple of weeks (see below).

I actually have several extensions to improve soft credits; several seem relevant to your situation.

Ready Soon

Instead of the "Send to Soft Creditee" extension, in CiviCRM 5.23+ I've added support for firing a hook on Soft Credit creation/update.  I also added support for "soft credit created/edited/deleted" in CiviRules 5.12+.  Both should be released in early March 2020.  Once both are released, the "Send to Soft Creditee" extension will be obsolete.  You can also get this functionality today by applying the patch linked here and downloading the "master" branch of CiviRules.

Currently, all these tokens don't actually operate on the contribution selected, but on the most recent contribution of the contact who made the donation.  This is due to current limitations in how custom (non-contact) tokens work, but Matt Wire is actively working on a solution.  I work around this by sending the thank-you out immediately via CiviRules.