the plan as you see it works well, the only possible issue is that recently the line from Milan to Brig has been subject to erratic engineering work which can make planning complicated. But if the trains are running to suit your plan then go for it. And if the weather to the west of Brig isn't too good on your free day you can head east to Andermatt, which is a very scenic ride and part of the route of the Glacier Express (but use an ordinary regional train). The town centre is relatively flat because it is built on the plain of the river Rhone, which is why it was almost wiped out by a massive flood in 1993, Hotel Victoria will have photos - it was lucky to survive. But the last time I was there I used the Good Night Inn, which is a simpler style but comfortable and good value. https://good-night-inn.worhot.com/ This is at the opposite end of the shopping area from the station but still walkable. The restaurants are mostly in that area, plus a couple in the side street that heads to the Stockalper Palace entrance. My last visit was during the same time as yours and the centre was snow free (ankle deep in villages higher up!).
But I wouldn't jump straight on the direct train to Freiburg - this runs through the modern base tunnel under a lot of the alpine scenery. If you set off about 50 minutes earlier you can use a regional train via Kandersteg on the scenic route, get off at Spiez and wait for the Freiburg train to catch up. The scenery is worth the earlier start. https://www.travelswitzerland.com/experiences/loetschberg-scenic-route/
If the MIlan - Brig trains are screwed up then Bellinzona isn't bad - very different in style but still handy. You can go west to Locarno by train - in fact you can beyond that back into Italy on the narrow gauge train which has some good views. Or you can go on the famous swiss post bus via the San Bernardino route - but the most spectacular option over the high pass is closed during winter and the bus will use a tunnel. And like Brig, the international fast trains towards Basel will use a long boring tunnel, but you can use the classic route's Treno Gottardo direct to Basel https://unterwegs.sob.ch/en/pages/trenogottardo
