Leaving est implicit is common, especially in succinct sayings like this.
Punctuation works differently in different languages and classical Latin had almost none.
It is good to remember that all punctuation and capitalization in classical texts are due to much later editors, not the original authors.
Supplying a comma makes sense here.
To me the most ...
Your question appears to me to be entirely appropriate for this site. Mottos and dedications (and tattoos) are regularly featured here.
That said, your phrase is not correct, as the adjective you're looking for is splendidus, -a, -um and not splendidis. It should therefore be:
Splendida mens mea
(You asked if your proposal was “a proper construction.” Well,...
Quin behaves very differently in independent and subordinate clauses.
In the Terence quote quin accipis it is used independently and means roughly "why not".
In the sentence in question it introduces a subordinate clause and does not mean "why not".
A&G discuss the use of subordinate clauses introduced by quin and quominus at some ...