Outsiders CAN buy the D-ticket. But you won't get instructions from me personally (have never bought one) so you should look to the top expert for details: The Man in Seat 61 explains the options.
Also, there are individual forum members here who can provide help using other strategies - I think the Mainz and Hamburg transit authorities have been mentioned as sources. Do a forum search. Here's one of those threads:
https://community.ricksteves.com/travel-forum/germany/d-ticket-easy-to-get-and-easy-to-use
A D-Ticket should make local transit much simpler for June. Without a May D-Ticket, May journeys by local transport aren't covered, as you know. So for your May DAY 1 FRA > Bacharach journey, you will need a ticket (or use a railpass day if you have a GRP.) You can look up FRA > Bacharach train schedules at the DB site, but you should not try to buy this ticket in advance online.
A search at the DB site will likely turn up regional trains (no ICE or other high-speed trains) for this trip - so despite the 1-hr travel distance, this is still a "local" (Nahverkehr) trip in Germany because it uses just the regional trains. DB can sell most regional train trips, but this is one of those regional train trips that DB can't handle...
A few of the journeys on your results page might show a price (probably €59 for two adults) in the far-right column. If you then hit the red "continue" button for that journey, you will get additional purchase options. Please don't pull out your credit card.
For the unpriced journey, hit continue and you will see a bunch of confusing tickets with German names and messages saying purchase is not possible. *Back to this in a minute...
Here's the deal: FRA > Bacharach lies marginally within the "RMV" zone. RMV is the area's transit authority, and only the RMV can sell tickets (train, bus, subway, whatever) for local-transport trips within this zone. Any prices (like the €59 price) you see at the DB site is for a journey that makes a DETOUR outside the RMV zone - DB has the authority to sell regional tickets that cross zone boundaries. (Click on "details" for those journeys which have prices, and you will see that the multi-train sequence has a stop in Bad Kreuznach, which lies outside the RMV.)
SO... What you don't need for your FRA-Bacharach trip is a detour or more changes of train or a longer trip or a higher price than you need to pay!
Back to that results page with all the ticket choices. The FIRST one, "Einzelfahrt Erw.", shows the cost of your one-way trip: €15.30 each. Transit authorities like the RMV have their own rules... Any one-way RMV must be used immediately, which is why you should buy it just prior to boarding, lest you be caught with an expired ticket. This is easy to do at a ticket window, a little trickier at a ticket machine, and probably easiest using the DB app.
How many other times during May will you be using local transport? Will the trip to Cochem also be in May? Will you use the train to get to/from the boat docks for a Rhine cruise? Will you travel to Burg Eltz? Clearly, it would take only a few days for an additional D-Ticket for May to pay for itself. Then you won't have any local transport tickets to fuss with.
Back to the schedule... If you are like me, you'd want a DIRECT train to Bacharach without making a change in MAINZ or Ingelheim. The RE trains that leave FRA's Regionalbahnhof station every two hours or so are the ones to use with your RMV tickets, or your D-Ticket.