Question – Should I choose a niche in print on demand or just create what I like without a niche?

Of course you are free to do as you choose, and should in fact do just that.

But, as you are seeking advice, mine is:-

Choose a niche; possibly several, if you keep them separated.

You will find it far easier and far more effective to promote to a/each niche in a highly targeted manner. ‘One-size-fits-all’ is far less likely to produce great results.

Choose a/each niche on the basis of having a strong affinity with it; knowing its frustrations and its secret desires.