Is there a way to say "I respectfully thank you" to familiar elders? From here.

Honorific Verbs
Like nouns and proper nouns, some Chinese verbs can also be complemented with an honorific modifier. For example, the verbs 告gào (&quot;to tell&quot;), 还huán (&quot;to return&quot;)，陪péi (&quot;to accompany&quot;), 劝quàn (&quot;to urge&quot;)，送sòng (&quot;to gift&quot;) can be complemented by the honorific prefix 奉fèng~ (Lit. to offer respectfully) to form the more polite versions 奉告 (I respectfully tell you), 奉还 (I respectfully return to you), 奉陪 (I respectfully accompany you), 奉劝 (I respectfully urge you), and 奉送 (I respectfully gift you). Another example is the honorific prefix 恭gōng~ (Lit. &quot;look forward to respectfully&quot;), e.g., 恭贺 (I respectfully congratulate you), 恭候 (I respectfully await you), 恭请 (I respectfully invite you)，恭迎 (I respectfully welcome you). The addition of the honorific prefix turns these verbs into a politer version. Unlike adverbs, the prefixes are often verbs themselves, and the compounded honorific verb functions as a single language unit (i.e., a single verb). Other common prefixes for verbs and verb examples are summarized in the verb complements section below.

Is there a way to say &quot;I respectfully thank you&quot;? I've always feared that 谢谢你 was too casual for elders. And 我感谢你 too awkward to say to elders that are &quot;basically family&quot;?
You could say 谢谢您 instead.
When you really want to be super respectful, especially to some respected person who you are not quite familar with, you could go with the following

承蒙关照，不胜感激。
厚情盛意，应接不遑，切谢切谢
感荷高情，非言语所能鸣谢

There are many more here which you can check out.