Experienced Devs

Just a note by setting up a 30 hours home project you effectively removing "people with lives" from your hiring pool. People who can do a 30 hours either have a lot of freetime currently, or code after the job. And if you really want those people in the team then go ahead, but you are missing on 8 to 5 crowd and that is a very good and diverse talent pool. From my experience 8 to 5 minded people are very good in solving tasks in sustainable manner. They just don't have time to fuck with the system and doing effective "dont-call-me-at-night" solutions.

if you are doing a lot of interviews you need a common set of questions and measures and this take a lot of time and effort to setup.

Personally I would suggest to setup interview as a two parter first ask some theoretical questions and then ask to create a simple code with simple problem related to the questions. This helps to find out if people are really understand what they talking about. This again require a lot of thought to setup an to have small practical tasks relevant to the questions.

For example in most recent interview I asked candidate about algorithm complexity, data structures, garbage collection and then asked them create a simple dictionary to store a hierarchical structure. This helped to see if candidate knows what he is talking about and can use his knowledge in practise. I have seen a lot of people without good theoretical knowledge, but they create a code that is good and working despite their gaps and other way is also correct people have a good theoretical knowledge but fails to apply it in practice.

So figure out who you are searching for. Create an ideal checking solution for their skills and start combing the desert. There is no shortcuts in hiring, sadly.