How hard is it to transition to the Army Reserve from the National Guard?

For officers, its a much more streamlined process.
For enlisted, not so much, but it varies by state (TAG). Promotion opportunities are great for E-5, E-6, E-7 and thru O-5 for the O ranks. Warrant officer is wide open in most WO MOSs. You may have to reclas if you are coming from combat arms unless you are interested in instructor or Drill Sergeant positions which are MOS immaterial. The USAR also has quite a few Branch immaterial positions for Officers to include Command positions as well as bn/bde staff level positions.

AGR postions are available, mostly in support roles for enlisted, most units have a 92Y, and 42A and a Training NCO.(MP units will need 31Bs, Medical Units will need 68W, Transportation Units will need 88Ms) Recruiting and Retention (79R and 79V) are always in demand.

For officers, LOG, AG, QM, MPs are usuallly needed. Be MOS Q, NCOES qualifed and willing to accept an assignment and you will have a better chance.

HRC accesses a couple of thousand USAR AGR Soldiers every year.........

I have done this and would be happy to talk to you at greater length in a non-public forum if it is something you are seriously considering.

As a general rule:
If you have a (NG) service obligation remaining, it is as easy or as hard as your state PTB want to make it. If you have no (NG) service obligation remaining, it is as hard as falling off a log. As an O, you can literally write a letter stating that you are resigning your NG (but NOT your reserve) commission effective such and such a date. The admin process is a bit more than that, but not much. Then again, the admin process is not your problem.... Of course, that's kind of a scorched earth approach, and shouldn't be taken lightly. But I guarantee it works!

As an O, you likely will NOT have to reclass. Remember, you have about 2 years to become DMOSQ. Which is about the length of the average position.... Not an issue in the company grades. The bigger issue, based on what I have read of your writing, is going to be the branch structure. As an IN Officer, you will be in the same boat as me (AR) - you only "qualify" for Branch Immaterial positions (unless you are willing to commute to Hawaii for drill!). While not a show stopper (I held a BN command and a number of other decent positions over the last 10 years), it does limit your options at the field grade level. If you were to move over, I would strongly suggest you consider a branch transfer prior to MAJ. It will make life easier. Or adopt a FA. I'm FA30, which definitely makes me more marketable.

Promotion opportunity, especially at field grade ranks is better - at least if you are motivated. There are at least five reasons for this:
First, you will be eligible for promotion on average of one year earlier (don't take my word for it, peruse the last 4-6 Reserve component FG promotion board milpers messages and check the DOR eligibility requirements for the NG and USAR populations.
Second, you are competing nationally. This is a bigger deal in small states than in large ones, but each state board may be looking at filling a FEW vacancies, while the USAR boards are looking at all anticipated nationwide vacancies.
Third, you don't NEED a position nearby to get promoted (or further your career in general). You can accept a position in another state, as long as you are willing to commute. I would expect this to be bigger deal with the smaller states. Moreover, it is much easier to move from a TPU unit to the IRR for purposes of accepting a promotion, if a slot isn't immediately available, and then come back into TPU.
Fourth, there is only ONE board that needs to be passed for promotion, not two.
Fifth, The structure of the USAR is somewhat top heavy. There's more room at the top. Additionally, there are a lot of vacancies at higher ranks.
(there is a sixth, dealing with the OBC, but that could always be just sour grapes, so I'll leave it out. ;-) )
I can't speak to AGR, as that has never been my direction. And I'm just too damn old. One difference with Mil Techs is that USAR ones do NOT wear the uniform during the week.

Mobilizations (stateside or overseas) are fairly easy to come by as an individual, less so as a unit. At least right now. Also, my experience is that commands are more willing to support individual mobilizaitons. That will vary by state and commander, but there is less of a sense of "this Soldier is MINE", less of an instinct of "we can't do without Joe."

The Army Reserve has vacant position posted, Retention NCO should have access to see these open positions. The best way is to target those openings and discuss the fit with the gaining Command, especially if there is a reclassification or school that must be paid for by the command. You need to think about how the Guard unit Command will react, they must release you. Transition from the Guard to reserve takes time.
The best situation is where there is a win/win.