Bayern Ticket

Frankfurt and Dresden-Leipzig are outside Bavaria.

https://www.bahn.com/en/offers/regional/regional-day-ticket-for-bavaria

Unlimited number of trips within Bavaria in one day
Travel with up to 5 persons for only EUR 8 per additional passenger
Valid from Monday to Friday from 9 am to 3 am the next day
On weekends and on public holidays the ticket is valid from midnight
Children up to 5 years always travel free of charge

But there are offers like the regional tickets for Germany.

https://www.bahn.com/en/offers/regional/day-ticket-for-germany

Travel throughout Germany for as little as EUR 42 with the day ticket.
Swimming in the Baltic Sea, a stroll through Munich or a hiking tour in the Black Forest: Discover all of Germany for a whole day on local and regional services with just one ticket.
Advantages:
Book 1 ticket and take up to 4 people with you
Every additional passenger only pays EUR 7
From Monday to Friday from 9 am until 3 am on the following day
On weekends and on public holidays the ticket is valid from midnight
Children aged 14 and under often travel for free with their parents/grandparents

The Man in Seat 61 website has lots of additional hints and information.

https://www.seat61.com

https://www.seat61.com/train-travel-in-germany.htm

quer-durchs-land-ticket is like the Bayern ticket but covers all of Germany

Land is a German state; Länder is the plural, states. Länder-Tickets are all day passes for states in Germany.

There are Länder-Tickets for travel in every German Land (or combination of Länder). They are for unlimited travel on regional trains in that Land for a day.

Bayern is the German word for Bavaria; the Bayern-Ticket is valid for travel on regional trains only in Bavaria. There is also a ticket, QDL-Ticket (Quer-durchs-Land-Ticket, or "entirely through the land"-Ticket) for all regional trains across Germany for a day.

Kahl-am-Main is only about 15 miles from Frankfurt Hbf (as the crow flies), but it is in Bavaria, so a regional ticket from the Frankfurt metro (RMV) will get you to Kahl. From there a Bayern-Ticket will cover you on regional trains to anywhere in Bavaria.

Kahl is NOT a suburb of Frankfurt! It isn't even close to Frankfurt. It is more a suburb of Aschaffenburg.

OK, Kahl is not inside the RMV, i.e., you cannot get to it on the Frankfurt S-Bahn, but it only take about ½ hour (28 - 33 min) by direct train from the Frankfurt Hbf to Kahl. And it's still an RMV fare (not a Bahn fare), 8,80€.

It's about 15 miles as the crow flies from Frankfurt Hbf to Kahl. I call that pretty close if you are going to Munich or Nürnberg.

You can purchase the Bayern-Ticket at the Frankfurt Hbf along with the RMV ticket to Kahl and use the Bayern-Ticket once you get to Kahl.

This argument is silly. Kahl is 28km walking (not 15 miles), 37 driving. Mainz is closer but also is not a suburb of Frankfurt. Neither is Wiesbaden or Hanau or Darmstadt. All on the RMV line. This does not make them a suburb.

I like this Deutsche Bahn template for checking rates and routes on DB:

https://reiseauskunft.bahn.de/bin/query.exe/en

Put in the to & from and check Local Trains if you want the cheaper (slower) options. I put in Dresden to Nürnberg with an intermediate stop in Leipzig. It is about 7 hours, not accounting for much stop in Leipzig. You can use Nuremberg as well.

Land is a German state; Länder is the plural, states

That is true, but it's not the whole truth. :-) The word "Land" is ambiguous and can also mean the whole country - which, apparently, is what is meant in the case of the "Quer-durchs-Land-Ticket".

Kahl is 28km walking (not 15 miles)

You do understand the relationship between km and miles (I hope)? 28km IS 17.4 miles, that's only 2.4 miles more than what I said, 15 miles, and I did say "as the crow flies". I don't think a crow would walk 17 miles, around obstacles, when it could fly direct, 15 miles.

But the point of my comment, since the OP asked about being able to use a Bayern-Ticket to go from Frankfurt to Nürnberg, was that, although he can't use a Bayern-Ticket for the entire trip, Frankfurt to Nürnberg, he doesn't need to buy a more expensive QdL-Ticket all the way to Nürnberg. He only needs a ticket for a short distance, Frankfurt Hbf to Kahl am Main, and then he can use the Bayern-Ticket the rest of the way.

However, if he knows he can commit to a specific train, he should also see if he can snatch a Sparpreis ticket for a faster (2 hr) ICE from Frankfurt to Nürnberg. With advance purchase one of these could be as low as 17,90€, less than either the QdL- or Bayern-Ticket.

It still is not a suburb, which is my argument.