Tax is now 4%. You can do the entire registration through a mailing service like Americas-Mailbox. www.americas-mailbox.com
. You need to register before any temporary tags expire. As to having split drivers license/registration if you are stopped for a traffic infraction might take some explaining. If you are stopped in Texas or SD you might have a lot of explaining to do any may get a ticket for not following their individual residency requirements.
Last question is where do really want to have your domicile? Voting, health care, taxes if any, etc. Not a decision to take lightly. Especially the insurance if you don't have healthcare through an employer or are of age for Medicare.