How to travel from St Gallen to Neuschwanstein castle? Navigating castle w/ physical limitations?

Hello:)
1) I am seeking advice on the best way to travel from St. Gallen, Switzerland to Neuschwanstein Castle, Germany in May.

I will be traveling with a Eurail pass, but the train looks like it will be 5+ hours according to Rome2Rio app and goes way out of the way. Bus is not better, about 6 hours by bus.

However, it is only 2 hours by car. I am wondering if anyone has hired a private car & guide to drive this route and stop at any scenic places along the way? Or even just a driver to get from point A to point B?
Or possibly should I look into renting a car for a few days from St. Gallen? Not sure if there would be a place to a) park near the castle; or more importantly, drop it off in that area?

I will be spending 1 night at Neuschwanstein, then I will be heading to Salzburg. It looks like the train from Neuschwanstein to Salzburg is approximately 4 to 4.5 hours, driving is 2.5hrs. (I am not able to stop at Munich during this trip due to timing for other destinations, but train does go via Munich).

If I were to rent a car, I would possibly rent it for 2-3 days to go from St Gallen to Neuschwanstein Castle, stay overnight there 1 night (or possibly 2 nights) then go on to Salzburg.
Note I will be arriving in St Gallen in the afternoon from Lucerne that same day, via the Volalpen Express train, before departing for Neuschwanstein. I don't plan on spending time in St. Gallen, I would rather the day be spent getting to Neushwanstein and enjoying that area.
2) Traveling with 19yo with physical limitations (doesn't require a wheelchair for normal days, but gets one at the airport and theme parks due to POTS/Autonomic Dysfunction and fatigue issues). Any suggestions for shorter hikes or other things to do around Neuschwanstein Castle to enjoy the natural beauty of the area for someone who has physical limitations? Also navigating the castle with physical limitations?

I remember the area surrounding the castle being so beautiful when I saw it in 1990 and did 2 or 3 hikes right around/near the castle. I will be traveling with my 19yo who has POTS/Autonomic Dysfunction, so she gets fatigued easily in general, and especially with slopes. I am slightly worried about the walkway to the castle, I remember it being very steep. Any suggestions on navigating the castle and ways to enjoy the beauty of the surrounding area with physical limitations is greatly appreciated! (ie: could we rent a scooter or wheelchair to access getting to the castle? is there an accessible tour of the castle?)
I know my daughter will really like this castle, I want to figure out how to do it in a way that the disability doesn't wipe out the beauty of the experience:)

Thank you so much for any suggestions!!