‘Resting’ is the better of the two because ‘myself’ is redundant. But in most contexts it is still unnatural. I’ll rest for 30 minutes. I’ll take a 30-minute rest. ‘spend time resting’ could be ok I guess if you are talking about budgeting your time in the future.

" i will spend 30 minutes resting" is more grammatically correct. This is because in this case you are both the one 'doing' the 'resting' (verb) and the recepient of the action. Adding 'myself' would be repititon. Unless you were to be more specific to say " I will spend 30 minutes resting my head" or refering to another subject other yourself. I hope this helps

The grammatically correct sentence is I will spend 30 minutes resting.

The first one will be better. You have already used "I" so it's clear that you are referring to yourself, so "myself" is not necessary. 休息比较好的expression可以是：take a break "I will take a 30 mins break" Hope this helps!

The better sentence is the second one, because you have given your sentence a subject and a predicate. So the, (I WILL SPEND 30 MINS), is your subject and the, (RESTING MYSELF), is your predicate. Remember the predicate always starts with a verb. I hope this is useful :)