I have used the DevDemon form addon since EE2 but its basically dead at this point. I reported more than one bug to the developer over a year and they don't bother to fix anything. I don't think it will be upgraded to EE4 either and the last update was over a 1.5 years ago.
I would just buy the one from Solspace but it has something missing that I really need and most forms have today.
The DevDemon form has this but its extremely buggy and at this point not even working with EE3 anymore correctly. Neither is the multiform feature and I don't think there is future for it with EE4 either. I only use the module for more advanced forms as the build in form functions from EE work fine for basic things.
Is there any alternative for more complex forms today? Pricing is not a problem.