"I get the impression that getting to St. Goar or St. Goarshausen and riding down to Rüdesheim is the best and fastest way to tour the river. I assume getting a car service or uber to take us to the town and then jump on a cruise is the fastest way to go. Any advice on how to accomplish this trip is greatly appreciated."
There's been no advice here so far on how to arrange a car service/taxi/uber for your outing. Maybe that's the best plan. You could leave FRA right after you exit security, and your service would take you back to FRA right after your cruise. No matter what time you leave FRA, there's no train schedule to work around, no connections to make or miss - only the cruise boat schedules.
So far, it seems like no one here has any personal advice to offer on the topic of a dependable personal driver. I know I don't.
There's a recent thread on private transport at the Tripadvisor forum where specific company names are mentioned. Maybe that will be helpful:
https://www.tripadvisor.com/ShowTopic-g187337-i301-k14475949-Airport_Transfer_Services_from_FRA-Frankfurt_Hesse.html
Whether by train or by private driver, you should...
Begin the cruise either in Bingen or Rüdesheim, the closest Rhine towns to FRA within the scenic zone.
Choose a Cruise - either #1 or #2 below. All take 1.5 hours or less. B-R times below are published for 2024. K-D times are from 2023, not yet published but likely the same or similar in 2024.
1) The classic Rhine Cruise ("Loreley cruise") takes you one-way downstream and ends in St. Goarshausen or St. Goar (across the river from each other.) You will see more of the river with this cruise, but your return trip to FRA will be longer. Driver must meet you downstream.
K-D from R'heim at 9:00 (or 11:00); from Bingen at 9:15 (or 11:15.)
K-D terminates in St. Goarshausen at 10:45 (or 12:50.)
B-R from R'heim at 11:00; from Bingen at 11:15
B-R terminates in St. Goarshausen at 12:45
2) Castle Cruise: a round-trip cruise of castles closest to FRA with B-R. Your driver can pick you up at the same place you were dropped off.
From R'heim at 10:30; from Bingen at 10:45
Terminates in Bingen at 11:40; in R'heim at 12:00
Driving time to either Bingen or Rüdesheim from FRA = roughly 40-45 minutes according to viamichelin.
So you have a variety of cruise start times... 9:00, 9:15, 10:30, 10:45, 11:00, 11:15. But you don't have to choose one right now. Once you've met your driver, you can determine which cruise time is the most feasible and which cruise dock to use.
Let's say you land at FRA at 7:00. Exit security, meet driver 8:00. #1 option looks doable...
- FRA > Bingen docks w/ driver (45 minute drive + 20-min cushion) 8:00 - 9:05
- Loreley cruise Bingen > St. Goarshausen 9:15 - 10:45
- St. Goarshausen > FRA return trip w/ driver 10:45 - 11:55 (viamichelin estimate)
- With 8 hours between flights, 3 hours of waiting time at FRA remain.
Let's say you land at 8:00. Exit security, meet driver 9:00. #2 option looks doable...
- FRA > Rüdesheim docks w/ driver (45 minute drive + 20-min cushion) 9:00 - 10:05
- Castle Cruise Rüdesheim > Rüdesheim 10:30 - 12:00
- R'heim > FRA with driver 12:00 - 12:45
- With 8 hours between flights, 3.25 hours of waiting time at FRA remain.
I don't see a plan like this, with time cushions like this, as risky - just as long as your driver is reliable.