NP-hard (3-)SAT variant with $n$ clauses and $f(n)$ variables

Question

With the purpose of proving my problem NP-hard, I'd like to reduce from a SAT variant (which of course should remain NP-hard) in which not two parameters are present (typically $n$ clauses and $m$ variables) but only one, so $n$ clauses and $f(n)$ variables, for some function $f$.

I've tried to look up 3-SAT (SAT with exactly 3 literals per clause) with exactly 3 occurrences of a variable, as this would imply I'd have $n$ clauses and $n$ variables which would be perfect. Alas, this paper proves 3-SAT with at most 3 occurrences per variable is trivial, including my specific case of exactly 3 occurrences.

All hope is not lost though, as in the same paper, it is proven that 3-SAT with at most $k$ occurrences per variable, with $k > 3$ is NP-hard. I'm looking into the details of the reduction to see if by chance they have proved the specific case of exactly $k$ occurrences per variable to be NP-hard (which would extend to the general case of at most $k$ occurrences per variable), but wanted to ask this question here while searching:

Does there exist an NP-hard SAT variant with $n$ clauses and $f(n)$ variables?

EDIT: the answer is yes, there exists such an NP-hard SAT variant, see this post, so I'd like to modify this question just for the sake of it:

Does there exist an NP-hard 3-SAT variant with $n$ clauses and $f(n)$ variables?

(As a reminder, In this post 3-SAT means exactly 3 literals per clause, as opposed to the more common meaning of at most 3 literals per clause, used in the solution for my first question.)

Answer 1

General 3-SAT is reducible to 3-SAT with $n$ clauses and $3n$ variables, as more variables would mean one isn't even present in some clause and can be removed, which can be done linearly.

Conversely it's also equivalent to 3-SAT with $m$ variables and $m(m+1)(m+1)/6$ clauses, as otherwise some clauses would be redundant, and again this can be checked in linear time.