My understanding is that by doing QCDs first, only the remaining amount needed to satisfy the RMD is taxable. If you do regular distributions that satisfy the RMD before you do any QCDs, you'll owe taxes on the full amount of the RMD, with the QCDs just being extra distributions that aren't taxed.
If you need the full RMD for your regular spending, it doesn't matter, but if you don't need the funds, it makes sense to use QCDs to lower the amount of taxes you'll owe.
As far as why the RMD must be satisfied before any Roth Conversions (Or IRA rollovers), I've read that it is solely to make sure the RMD is clearly satisfied and to avoid any confusion with records.
I can foresee a situation where the order would be quite important. Suppose your calculated RMD is $60k, and you take that in January. You don't need the money to live on; you're just doing it to satisfy the IRS. So you pay the marginal tax rate on 60k. In June, your church's steeple is hit by lightning and burns. Emergency repairs are needed. Being a good guy, you make a $15k QCD to the church to effect the repairs. In this situation, due to the first dollar rule, you will not get the tax benefit off having the QCD offset part of your RMD. If you are in the 22% bracket, you will pay $3300 in unnecessary taxes.
Regarding the above, the ordering of distributions doesn't matter for taxation.
If you take out $60K in ordinary distributions and $15K in QCDs, your tax bill will be exactly the same regardless of the ordering (or size, or number) of the distributions. At the end of the year, you'll get a 1099-R with $75K in box 1, and you'll subtract the $15K and notate QCD on your tax return.
Now if Gumby is implying but not saying that the church steeple guy would have only withdrawn $45K in January had he known the future, then that is a different kettle of fish. You'd be right in saying he wouldn't be able to stuff $15K of that January withdrawal back into his IRA once withdrawn. So that is a legitimate timing consideration, but it isn't an ordering consideration.
The "rule" about completing your RMD for the year before doing a Roth conversion for the same year is a commonly accepted Urban Legend.
I say that because I found nothing about this in the IRS publications I searched a year or two ago, including the obvious Pub 590-B.
So, yeah, I'd like to see something from the IRS or Treasury department websites that addresses this issue.
I too like to have definitive and authoritative guidance rather than what SGOTI or a blog page posts.
Occasionally the BH people point out to me that the IRS publications aren't actually tax law, and you can't officially rely on them for 100% proper tax preparation. Even though 99.99% of us do. The actual tax law is Title 26 US Code, and then there are also IRS / Treasury regulations (in the CFR - Code of Federal Regulations) that effectively have the force of law.
My point is that even if it's not in 590-B, does not mean that it is an urban legend.
I personally don't think it's true, but I do think that pub 590-B could be improved by clarifying the ordering question.
Anyway, although we're talking about ordering of RMDs and QCDs, here is a link to a BH post where they talk about the ordering of RMDs and Roth conversions, which is a similar but distinct topic:
https://www.bogleheads.org/forum/viewtopic.php?p=7222461#p7222461
It's possible that the link there points to some stuff which might shed light on the ordering of RMDs and QCDs. I haven't looked.