If your Munchdew doesn't eat fast enough, you don't have enough Munchdews. 😉
But jokes aside, pre-Alfheim you are somewhat limited in your mana generation and transportation options. And pre-End your automation options are somewhat limited as well, since you miss out on redstring and corporea.
But as far as reasonably simple farms go, if you are anywhere near a swamp, a Narslimmus surface slime farm within a slime chunk could be an option. Swamp slimes are accepted by the Narslimmus flower, if the chunk they spawned in happens to be a slime chunk.
Also, Entropinnyum is always a good way to make a lot of mana reasonably quickly, if you can provide the resources for the TNT.