which of these train tickets do we buy in advance?

Info: Karlsruhe to Salzburg is Deutsche Bahn.

Planned date of travel?

All of them.

between may 15 and may 28th of 2020.

• All TGV tickets need to be bought in advance, discounts for advance purchase and all tickets are train-specific including a reserved seat.
• Not sure about Paris to Bayeux.
• Strasbourg to Karlsruhe, not all trains a TGV or ICE. If you choose the local trains, no reservations needed and it may be cheaper.
• Karlsruhe to Salzburg, DB (German Railways) ICE. Yes, discounts for advance purchase.
• Salzburg to Venice and Venice to Rome, yes buy in advance, discounts for advance purchase.

For more info see this webpage: https://www.seat61.com/Europe-train-tickets.htm

Edit: Just seen your new post "between may 15 and may 28th of 2020.". Way, way to early. Most cases ticket sales open 3 months in advance. Plan to buy in February 2020.

Pretty much what I thought. Strasbourg to Karlruhe is close enough we might take the bus.

thanks

Book all of them in advance, even Paris Bayeux. SNCF pricing changed this year, there are now some good advance deals to be had on that route. But, for Paris-Bayeux, 90 days is overkill. If it helps solidify your planning, 2-4 weeks will do.

"Strasbourg to Karlruhe is close enough we might take the bus." - Why? Slower, less comfortable, probably less frequent and possibly stopping at an inconvenient location not in the city centre.
IMHO

just depends on the time table of when everything is leaving

Its 40 minutes on a direct TGV or ICE, 1 1/2 hours with local trains and at least one change. Bought well in advance, they are a bit cheaper than local trains bought day of travel, 19.90 EUR vs 21.10 EUR pp.

About How much would a bus ticket be for that distance?

Frequent both fast and slow trains all day on the Strasbourg - Karlsruhe route.

Looking just a couple of days out there are €19.90 tickets available on some fast trains, including seat reservations taking 40 or 50 minutes; €21.10 on slow trains using regional trains only and taking an hour long including a longish change.

If you just want a walk-up fare you can have a fast train with no notice at €30.00.

If you want a slow trip take the regional trains. I don't think you'll get a better ride, or more frequent, by using a bus.

But what is the difference in price between the train and the bus?

I'm not ignoring your bus question - I don't know. I don't do buses - maybe somebody else does.

You might have a look at Flixbus and let us know what you find....

I just looked it up. Looks like it starts at $6 american and goes up to $11 or 12