What's the travel date?

You'll definitely change trains more than 3 or 4 times. Unavoidable unless you take the night train. It's not a big deal as long as you allow enough time.

The route via Paris might be the easiest but it'll be the most expensive. For example:

- IC6 Bern - Basel SBB 07:38 - 08:32

- TER Basel SBB - Strasbourg 08:51 - 10:09 -> very comfy train

- TGV Strasbourg - Paris-Est 10:47 - 12:33 10-20€

- 10 min walk, break for lunch

- Eurostar Paris-Nord - Amsterdam Centraal 14:55 - 18:53 32€ (passholder quota, book early)

Times subject to engineering works.

Thanks for your reply.

Week of the 19th August.

I was concerned the only reasonable option would be through France. I was trying to avoid the reservation fees. real pain considering how simple this journey usually is!

PS

I’ve just chalked up a route through Germany which has reasonable change over times

Switzerland - Baden-Baden - Banoff - Rastatt - (Frankfurt direct to Amsterdam)

I was wondering, however, if seat reservations are semi flexible? For example, If I turn up early in Frankfurt and a seat was free, could my reservation be used to take it? Its the only part of the journey where reservation is required.

Unfortunately not. There was a time where you could switch seat reservations, but alas. Times that are long gone sadly. 

You'll need to make them anew then, if there's any places left, and the ICE's to Amsterdam are quite popular and suffered from severe overcrowding last summer. So I doubt it that it'll be possible in sumer :/

That week there will be engineering works on the High Speed Line  in Belgium, so the Eurostars will be at least a half hour slower than usual, due to rerouting. 

19th August: ah then I picked up the right week! My schedule above accounts for the works in Belgium. Fortunately the works on the Frankfurt - Cologne high-speed line should be finished.

I'm not sure how reliable the buses will be honestly... (Rastatt - Baden-Baden). I guess it is better northbound as the train should arrive on time from Switzerland. In the other direction expect to wait 1h if the ICE is late.

I'd make 2 Frankfurt - Amsterdam ICE reservations. 3€ isn't the end of the world and these trains are packed year-round.

I am also a bit concerned about the busses. In the route I’ve knocked up there is a fair amount of changeover time as its either 10 minutes or 59 minutes in Rastatt, so I’m going to air on the side of caution.

With the hassle involved thought I don’t really imagine me or my partner will be wanting to do the entire journey in one go. There seems to be some quite cheap hotels in Frankfurt (even cheaper than the French reservations fees by €30-ish!), so I may just cut the long journey in two and have a night in Frankfurt, whilst spending less than going through France 

Frankfurt isn't the most exciting place ever. In that case I'd recommend Heidelberg or maybe Wiesbaden/Mainz for a night.

Further along the Rhine is possible too but likely more expensive than in Frankfurt!

I had heard Frankfurt wasn't amazing, but will make it very convenient for the follow on leg up to Amsterdam. All those places you’ve mentioned look a lot more interesting however, so I will have a think .

Now I do have another question unrelated to this situation, which I was thinking of starting a new thread about but I thought I’d ask this thread first.

As the Eurostar between Amsterdam > London is suspended, is my only option to book a reservation from Brussels > London,  then making my own way to Brussels? The reservation websites I’ve looked at (B-europe, raileurope) all state there are no available trains; but I know there are trains running people from Amsterdam to Brussels whilst the engineering work is ongoing. It would be nice to only pay one reservation fee rather than two or have to use the slower regional trains.

Yes you'd have to switch in Brussels with sufficient margin.

There are hourly reservation-free IC trains between Amsterdam and Brussels. Departure is usually at xx:25 but could differ if there are works. What's the travel date?

There are slightly faster Eurostar trains but they have a passholder quota and seats cost 27€... avoid them.

date would be 22nd or 23rd August. IC ones look okay on the DB website.

However, my hope was I would be able to get the Eurostar Amsterdam to Brussels part included in my booking back to London. But the websites don’t seem to provide this option. so I would have to pay 2 reservations fees if I wanted to get the Eurostar 2hr train from Amsterdam to Brussels. Even though usually you would only pay one fee from Amsterdam to London. Hopefully that makes sense.

That won't be possible I'm afraid. You could pay 27€ + 30€ but not worth it in my opinion.

Eurostar also often blocks off Amsterdam - Brussels sales to fill up the train with Amsterdam - Paris passengers (bringing more money).

I see. That is a shame!

Anyway, thank you very much for the help! its nice to have some other options than just a night in Frankfurt!

​​​​​​​​​​​​​​+also thanks to BrendanDB for your reply too!