What "like The Act" actually means
Similarity is not one thing
Two series can resemble The Act in completely different ways, and knowing which one you are after saves a wasted evening. There is tonal similarity — the same mood, pace and sense of humour, regardless of subject. There is premise similarity, where the setup rhymes but the feel does not. And there is the weakest kind, surface similarity: the same genre label, the same era, the same lead. Most disappointing recommendations are the third kind dressed up as the first.
The list below is built from what people who watched The Act went on to watch and rate highly, which catches tonal and premise similarity reasonably well and surface similarity by accident. So it is worth reading the top few entries with the question "similar how?" rather than taking the order as a ranking of quality.
Where to start
If you loved The Act for how it felt, start at the top and trust the first three or four — the strongest tonal matches concentrate there, because that is where the overlap in audience is densest.
If you loved it for what it was about, read further down instead. The middle of a list like this is where the interesting divergences live: series that took the same idea somewhere the original series did not go, which is usually a better evening than watching something that simply repeats it.