Looks really good! The Bernina route is my favourite Swiss railway :)

You could spend the first night in Strasbourg or Basel and the second in the Engadin Valley or near Lake Como.

Have a look at mandatory reservations on high-speed trains here : https://www.seat61.com/interrail-and-eurail-reservations.htm

Eurostar needs to be booked well in advance (popular route + passholder quota). Change in Lille (same station) or in Paris : Gare du Nord to Gare de l'Est -> 10 min walk.

Bernina Express : 36 CHF reservation for the (crowded) panorama carriages or reservation-free regular carriages at the front -> less crowded with pull-down windows. Also possible to take the hourly regional trains on the same route. You can see the driver's cab from the 1st class seats at the front (with a 1st class pass).

Questions welcome.

I would start by looking at this page on Seat61. It gives you a summary of the various options for getting from the UK to Venice, with some really good descriptions + suggested train times.

https://www.seat61.com/Italy.htm#london-to-milan-venice-florence-rome-naples

Unfortunately, Options 1 & 2 (via the Paris-Milan high-speed railway) aren’t possible at the moment because the line is blocked by a landslide.

Option 5 is the way if you want to include the Bernina Express. Highly recommended!

Then you could choose a different route for the return journey. Either the sleeper via Stuttgart, or the day route via Switzerland (much of it the same route as Option 5).

Many thanks for Raiderton & Thibcabe for your useful tips.  Much appreciated!

I think it’s romantical if you have rail trip with your darling or family, you will have time with the others on train