I'm going to suggest a hybrid approach. As you well know, your approach is right for almost any disease. Get the best heart surgeon, etc.
However on the medical side, that approach won't yield any different result with ALS. There are only 3 drugs approved for ALS and two of those were not approved in Europe because they likely don't do anything. We ALS patients are choosing drugs hoping they will give us a few more weeks or months. Of course, clinical trials can't split hairs this small. Every ALS clinic doctor knows these drugs.
On the procedure side, there are feeding tubes when we progress to the point of not eating. Then there are tracheotomy and laryngectomy when breathing and aspiration risks are the issue. These definitely extend life, unlike the drugs, but how long is unknowable because many patients die of non-ALS causes that become more likely because of ALS (decreased activity, etc.)
I have a feeding tube and got a laryngectomy in July. I went to Mayo Clinic in Jacksonville, Florida and I live four hours away. They have a comprehensive ALS clinic. Dr. Oskarsson was great. The overall ALS team was great. Like any team effort there were some standouts and one average member but overall it was great. I let them set up my breathing equipment supplier, feeding tube formula suppliers, wheelchair, etc., and all that has worked great. These suppliers are very responsive because they know Mayo is behind me. Mayo Jacksonville has hundreds of ALS patients at any given time.
The only downside is that on major procedures, her local doctors will not want to and in my case, refused to, even take out my laryngectomy stitches.
There are some great hospitals in St. Louis. One of them is bound to have a comprehensive ALS clinic. Staying in state solves all kinds of issues with setting up suppliers across state lines. One call to your ALS Association local rep can give you the complete rundown. If she is near there this solves even the later issues by getting all care there. Don't get hung up on the clinic not offering the research function. The odds of her hospital curing ALS are slim. She needs all the other functions.
Here is the hybrid approach. If she has to drive a long way for a clinic, do everything there except for procedures. I asked all my local doctors if this would cause an issue later and they all said no. No doctor wants to admit this is true but it a fact with US healthcare. They don't like these prestigious hospitals stealing lucrative procedures.
Know that going to Duke versus Mayo or any of the other clinics will not find you a miraculous artist that can meaningfully impact ALS longevity outside the known tools. What they can bring you is experience with many more patients, better supplier setups, etc., that small places might not be able to replicate.
So, keep it close if possible, if not do the hybrid approach.
One caveat, your approach comes back into play when considering more complex procedures like a laryngectomy for example. You want someone who does a lot of these. My doctor does 100 a year for example. But there will be someone in St. Louis with that experience.
The Les Turner Center for ALS at Northwestern may have the single best site on the web for ALS so I imagine they have a first rate clinic as well if that is doable for her. Plane travel becomes difficult later on so I suggest keeping it to a driving distance.
Hope that helps.
Doug