What advice would everyone offer (other than run a lot) to a warrant officer candidate?

As you are a mid-grade NCO, the recommendations won't be much different than what you already know about effective leadership skills unless you're that rare breed of NCO who never had a group of individuals working for them. Effective communications, empowerment, lead by example, etc.

On the technical side, WOs are the SMEs, but as a 915A you're not going to be expected to be the SME on 'fixing the vehicle' (you could be, but that's not the expectation) but rather be the SME on, and primary executor of, the organization's maintenance program, shop safety, etc. You'll still need to know the 'what' and 'how' of the wrench turners, but you're not going to be doing a lot of that unless you're very understaffed or don't have trained individuals.

Instead, your technical expertise is going to be focused on keeping up with new training, technologies (especially changes in the technical manuals), diagnostic procedures, etc. You may not know the answers, but you need to become a master of figuring out where to go to for the answers when it comes to obtaining parts for vehicles, who to turn to when your organic resources (people, equipment, etc) can't diagnose an issue, etc.

If you were active duty at a battalion or brigade level, then you and the XO will need to schedule a procedure at the MTF to be surgically joined at the hip. It's still going to be true to an extent in the National Guard, but it's going to greatly depend on your unit's OPTEMPO, maintenance agreements, etc.

However, whatever it is, you'll need to really have a fundamental understanding of all aspects of "how is the equipment maintained and repaired". If your vehicles are all at a state facility with contract maintenance that takes care of them, get to know the process, procedures, POCs, funding, building locations, etc. for it. If your unit takes care of them internally, what are the procedures? Do you have full-time staff that takes care of everything or is it a combination of M-day Soldiers and full-timers?
Who does what? How are replacement parts obtained? Where are stocks kept? Etc.

A lot of this you might already know (I'm assuming you're not changing states), or at least have been exposed to in the past as a 91H.

The list goes on, but you get the point - you'll need to get a comprehensive understanding of how the process works, from beginning to end and it's going to differ from unit to unit. Why? I always had a saying about the National Guard - if there is a best way of doing something in the Army, I guarantee you that I'll be able to find it in the National Guard. Conversely, if there is the absolute worst way of doing something, I'll probably find it being done that way in the Guard as well. Why? The National Guard is a collection of 54 independent fiefdoms tied together with policy, regulations, and funding.

I think the myth that warrant officers are invisible and nowhere to be seen is a misnomer (in the Reserves anyway). You could very well find yourself in a platoon leader position by default being slotted in a platoon without any LT's.

If you're in a situation at the company level as a WO chances are you'll not be surrounded by CWO mentors (and at best they will be 2's anyway). So you'll have to hit the ground running, and not be given near the rope 2LT's might be.

I had a maintenance section in my company with a maintenance WO. It was also one of my two sections without an LT. They figured their roll well enough and as long as company assets kept running they had it pretty good (all of my sections were like that really (food prep, warehouse, etc...).

I as a reserve company CO would be more incline to pull strong PSG's from platoons with WO's at the helm to help out LT's who are on their own.

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OH.............. Here is some actual significant advice you can carry forward regardless of everything else going on around you:
You write your OER. Spoon feed it to your rater (yes, they actually have to fill it out in the system, but give them comments they can cut and paste). It's not going to be like your NCOER, least of all your E5 NCOER's (which are more or less GO/No-Go). Make your rater reel in your overly optimistic OER comments. No one is going to bend over backwards fluffing up your OER.

Plus, you'll have to be the rater for many NCOER's as well. Be prepared for that.

Everywhere in the Reserves raters are rushing to get evals through HRC without getting kicked back. It's probably the most important thing in our USAR career and so many of us just blow them off.

****As a soon to be WO make sure our OER's are as glossy as you can make them from the very first one****

Never was fond of Warrant Officers as they always seemed scared of their NCO's.