Munich to Budapest

I think you have to make decisions about what you are able to be comfortable with.

I just did a couple of dummy bookings and could buy a ticket on
Monday, May 14 and Monday, June 3. I did this since your date is a Monday.

I could NOT buy a ticket for June 10 or after.

What that tells me is you are simply too early. Tickets for many train companies are simply not released or available to buy way far ahead. Sometimes it is 90 days ahead, sometimes 60 days, and in a few cases (looking at you, Renfe), not a lot of rhyme or reason we can discern. Typically a new schedule is released each year about June 10. I suspect this is what you are up against. But they WILL be available.

Whether waiting makes you more uncomfortable than you like, only you can determine. It is good you have an alternative.

Or fly? ......

I just sent an email to AAA to see if one of their travel agents could do this for me. Hopefully, they can, though I suspect they may run up against the same scheduling problem as described by TexasTravelMom above.

Thank you Mr. E and TexasTravel Mom. @Mr E. - I did contact Laszlo (?) at silver wings and referenced your name (and told him you said hello). His price was $1100 and he (in so may words) advised me to take the train.

It is April. You are traveling in August. Your deadline for buying tickets is August 26. Yes, you can just rock up at the station, go to the ticket window, request a ticket to Budapest, and travel.

Buying tickets in advance is something you do to save money. You do not have to fear tickets selling out. They don't. Seat reservations might sell out, but that only very close to the travel date. But you do not need to do this far in advance.
Trains do not sell out months in advance. If they did, they would be useless.

So you can safely wait till around June or so to buy this ticket.

Now regarding the weird messages you see on booking sites: DB has decided to make reservations compulsory for these trains (good idea in my opinion) but they have badly implemented it, as they have not a lot of experience with that. So the result is that the trains, and the fares are already in the system, but the seat inventory isn't. Normally you would then just buy a ticket, and sort out the reservation later, but currently the unfortunate situation is that the booking engine tries to find you a seat, doesn't find one, throws up its hands in the air, and gives you a not very useful error message.

Yes, you are spending far too much time with this. It's four months left until your trip, if you can't buy a train ticket four months in advance, the solution is not to try some strange booking site or look for other options. The solution is to wait.

Thank you, WengenK. You have explained it very well. What got me started on this most recent rant was omio.com. Going through a dummy booking on the date we wish to travel (Aug. 26), I tried various departure times (on that date). Some results (with direct connections) read "all booked up." Other (less desirable connections - i.e.,not direct) trains with transfers read "limited seats available." So that got my attention and caused this alarmism. Thank you (and to others) for the reassuring words. I haven't used trains in Europe since college and it is like I'm starting all over again.

Thank you very much (again), Badger. That seems to be the consensus. As stated above, omio.com set up the alarm bells for me in this instance.

Fly. It's $154 via Lufthansa and only 1.5 hours.

Thanks KGC. We tend to get to airports very early. When you add 2.5 hr arrival before flight and 1.5 hr flight time, plus hassle and time of getting to hotel from Budapest Airport, we could almost already be there by train. Thanks so much for researching this and for the helpful suggestion. Very much appreciated. I'm going to trust the other responses on this thread in which it is indicated that there should be train seats available.

I am with KGC. I dont do well sitting in one seat for more than 4 hours if I have options.

Accommodations to train station, plus a little early arrival because never been to that station before: 0.75 hours

Train to Budapest 6.5 hours

Disembark train at Nyugati station to town center by taxi 0.75 hours (probably no faster if you take public transportation as the metro underground is a bit complicated at Nyugati)
Total time: 8 hours.
Time sitting in one seat: 6.5 hours

Accommodations to airport, plus 2.5 hours early: 3:0 hours
Flight time: 1.5 hours
Disembark and get a taxi: 0.75 hours
Taxi to town center: 0.75 hours
Total time: 6.0 hours; might even be 6.5 hours.

Time sitting in one seat: 2.5 hours

It's a local flight, No need to do Customs, you're flying from one EU airport to another. You can show up 30 minutes before it leaves and get on the plane.

Thanks KGC and Mr. E. @Mr. E - You have broken down the total time analysis expertly. Much appreciated. I would not have known local time issues such as these. Two hour time savings by air. On the train side - it's sort of a "backdoor experience" in itself.

The choice of plane or train is often more about which experience you want and not time or cost. But never hurts to understand all the details.

Totally agree, Mr. E.