You might want to look at getting the Nightjet sleeper train from Brussels in at least one direction; it will save you having to make a really early start in Newcastle.
I agree, I would definitely look into this. Can be pricey but if you book ahead (booking can open up to 180 days in advance) some cheap advance fares (Sparscheine) are available on oebb.at. You can either cram into a 6-bed couchette, or pay extra for a compartment, some of which have ensuite showers.
Everything is up in the air at the moment obviously, but on the pre-Covid timetable, you could get the 12:58 Eurostar from St Pancras and arrive Brussels at 16:08. Departure time from Brussels is 18:04, and that two hours is good "insurance" in case of delays. If everything runs to schedule, you have time to grab something to eat and drink at the station before boarding the sleeper. You arrive in Munich around 7am the next morning.
Currently the sleeper only runs from Brussels twice a week (Mondays and Thursdays) but on other days you can get it from Cologne.
The Man in Seat 61 has a good overview of the Nightjet service here: https://www.seat61.com/trains-and-routes/nightjet.htm