Deutsches Bahn - allowed to board train at stop past your ticket origin?

I don't understand where your day starts or where you will ride your bike.

But in principle, you can board any long-distance train you are scheduled for with a SAVER FARE ticket at any scheduled rail stop subsequent to your planned boarding station (Mainz) for the specific train on your ticket. You cannot board a later or earlier train.

If you pay the flex fare, you can ride any trains you like that same day as long as you travel the exact same route.

I understand that you are asking a ticket legality question, but I have other questions:

Will you have a reserved seat? I just think it's less awkward to take possession of what belongs to you at the moment it's ... yours. If someone is sitting in your seat because you did not board, you'll have to ask them to move. And there's a risk that your bike spot will be taken because you weren't there. I'm not saying you're not "entitled" to the space, I'm talking about real life on a jammed high-season train.

I agree that my preference for relatively certainty is personal, but have you traveled enough to have had some negative experiences in this type of situation? Also note this DB FAQ:
https://www.bahn.com/en/offers/Seat-reservation-and-other

"How long is this [seat] reservation good for?"
A: "Your seat reservation remains valid until 15 minutes after departure.

Elsewhere: "You will need to purchase a long-distance bicycle ticket and a bicycle space reservation."

the train reaching Offenburg from Frankfurt a. Main at 18.30 is an ICE.

Frankfurt(Main)Hbf arr 16:44 dep 16:50 plt 9

Mannheim Hbf arr 17:27 dep 17:35 plt 4 (note no change)

Karlsruhe Hbf arr 17:58 dep 18:00 plt 2

Offenburg arr 18:31 dep 18:32 plt 1

note - no stops between Frankfurt and Mannheim

The two changes Mannheim and Karlsruhe slow train getting in 4 minutes earlier doesn't stop at Mainz.

So what about Mainz to Offenburg change Mannheim arr dest 1830? Let's look.

are you possibly talking about the 16:42 InterCity Mainz 14 minute change Mannheim to ICE arrive Offenburg 18:31?

If so, it doesn't come from Frankfurt but way up north and though the Ruhr (previous stops Köln, Bonn and Koblenz, and the next stop after Mainz is Mannheim.. I don't see how the riding a bike after Mainz would work.

Could you please specify the actual trains you're talking about? You have me confused.

Russ thanks very helpful, solves the question :)

Tim:

Will you have a reserved seat? I just think it's less awkward to take
possession of what belongs to you at the moment it's ... yours. If
someone is sitting in your seat because you did not board, you'll have
to ask them to move. And there's a risk that your bike spot will be
taken because you weren't there. I'm not saying you're not "entitled"
to the space, I'm talking about real life on a jammed high-season
train.

I agree that my preference for relatively certainty is personal, but
have you traveled enough to have had some negative experiences in this
type of situation?

Tim I've spend more than two years total traveling in Europe, probably 2.5 years at this point. As such have definitely experienced much of the likely negatives. But will say I've been quite lucky in train interactions! I will have a reserved seat, but if it's full and no longer entitled to me. Happy enough to stand by my bike. Neither leg of my journey is long, so no big deal.

If I ride past my origin station and it turns out another bike has taken my slot that's a different matter! A few times I've crunched in with an extra bike, and SO many times had an extra "pirate" bike (or two, or three) get on after I've boarded with a reserved bike. Generally then we cooperate - there's often sort of a "we're all in this together" feeling among cyclist on trains.

At any rate, the second train in my journey is the ICE with truly limited bike space - that one I should board at origin no problem.

Thanks gents, quite helpful both.

are you possibly talking about the 16:42 InterCity Mainz 14 minute
change Mannheim to ICE arrive Offenburg 18:31?

This is correct. Didn't seem necessary to be completely specific to get a useful answer, which I got above. General idea though Nigel is ride bike from Frankfurt at least to Mainz for a short but satisfying morning, farther along route of 16:42 IC noted above if time allows.

I might prefer riding my bike more than waiting around Mainz, but also definitely need to reserve for my bike. If something goes wrong on my ride it seems safest to have the ticket from Mainz (not so far from Frankfurt), not from some more distant more aspirational stop down the line.