Waring and Waring-Goldbach subbases with prescribed representation function

Let $h\ge 2$. For $A\subseteq \mathbb{N}$ write $$r_{A,h}(n):=\mathrm{\#}\{(x_1,\dots ,x_h)\in A^h\mathrm{\mid }{x}_1+\cdots +x_h=n\}\mathrm{.}$$ We prove a general probabilistic subbasis principle: assuming an asymptotic for a weighted $h$-fold representation sum over a basis $B$, there exist subbases $A\subseteq B$ whose representation function $r_{A,h}(n)$ has prescribed regularly varying growth. We apply this to $k$-th powers ${\mathbb{N}}^k$ and to $k$-th powers of primes ${\mathbb{P}}^k$. For $h\ge k^2-k+O(\sqrt{k})$, we show that every regularly varying function $F$ with $F(x)\mathrm{/}\log x\to \mathrm{\infty }$ in the admissible range is realized, with the expected singular series factor. In particular, there exists $A\subseteq {\mathbb{N}}^k$ such that $$r_{A,h}(n)\sim {\mathfrak{S}}_{k,h}(n)F(n)\mathrm{.}$$ Moreover, in the prime setting we obtain thin subbases $A\subseteq {\mathbb{P}}^k$ with $r_{A,h}(n)\asymp \log n$ for $n$ in the admissible congruence classes.