Thank them for this offer.
Thank them for remembering you.
Tell them that you think this offer sounds good, but it’s not quite right for this phase of your life. You don’t need to explain why, because you don’t owe them any explanation. If you want to explain, tell them it’s personal.
Tell them although now is not a good time for this offer. Maybe this situation will change in the future. You hope that when the right time comes, this offer will still be available to you if you need it.
Leave this offer as a potential opportunity for your future. Always consciously choose to leave more options for the future version of you.