There are other languages of similar difficulty (all constructed), but they are spoken even less than Esperanto.

That's a bit of a strong claim to make, considering that there are thousands of languages with more speakers than Esperanto, most of which you probably never heard of. My guess is that some contact language has similar difficulty, while having more speakers.

Pichinglis [1] seems like a good candidate: 70,000 to 100,000 speakers in Equatorial Guinea; formed as a creole from English and various languages of Western Africa, so you should get a high number of cognates similar to Esperanto and a simplified grammar. It might even help you learn other languages of the region faster.

[1] https://en.wikipedia.org/wiki/Pichinglis