The solution is simple: pay candidates for the estimated time the assignment should take, based on the median rate of the role they're applying for. Pay them regardless if they pass or fail the test. You should be able to avoid grifters looking for handouts if your screening process is any good.

But I don't think this type of evaluation is a valuable signal anymore in the age of AI. Anyone could nowadays finish your take home assignment in a fraction of the effort it would take them without AI. This might not be relevant after they're hired, since they would likely also use AI assistants, but for evaluation purposes you want to assess how someone thinks about problem solving, not how they generate the solution, and then confidently walk you through it.

The best interview style for software developers IMO is the code review challenge. Show them a piece of code from a hypothetical PR of a junior developer, and ask them to a) describe what the code does, b) point out any issues they see, and c) fix the issues. You can have several difficulty levels depending on the role and expected candidate experience, or you can cherry pick these from your own codebase. The benefits of this approach are that it's a simulation of a real-world task they would be doing on a daily basis, it's a collaborative effort and showcases their communication skills, while also allowing them to evaluate yours, it's much less stressful than white boarding and quizzing sessions though still involves some coding, and unlike take home assignments, it can be completed in an hour or 90 minutes at most.