I'd tune the ramp as a budget, not hand-placed counts. Give each enemy a cost (wisp 1, brute 4, splitter 3 as placeholders), then set budget(w) = base × growth^(w−1). For example, base 10 with growth 1.25 gives 10, 12.5, 15.6 per wave. Spawn until the budget runs out, and cap enemies alive at once so a new type can't swarm the fire by itself.
Two checks catch most unfair spikes: time-to-kill for each enemy in the wave where it first appears, and a dead-air check (if nothing is alive for more than about five seconds, the wave will feel boring).
Keep the wave table in a CSV (wave, wisp, brute, splitter, hp_mult) so you can tweak numbers without touching code.
For the wood question, logging warmth lost per wave against wood collected would show whether the loop is fair.