Eurostar Question - Non-London City to Brussels

You did not give the actual time frames. Some posters here know exactly when, over three months, Eurostar sharply increases advance prices. If you see (note that they vary throughout each day) a Eurostar price you like, buy it now. It will only go up. It is operates quite separately from domestic services. Note that Eurostar discount tickets are typically non-changeable and non-refundable. Allow substantial time for pre-boarding Immigration, and if there's a Brexit, check for new time estimates.

Note that the "any Belgian station" Eurostar option is often convenient, and sometimes can save money for distant places, like Bruges. But local rail in Flanders is so cheap and frequent (and unvarying in price) that the option is not a must-have. I believe there is no "any Limey station" option. Note that any Eurostar ticket to Brussels allows you to take any of eight or more per hour local railroad (not subway) trains to the other two downtown stations. So you don't care that Eurostar goes to the Midi station, not Centraal.

Even with Eurostar, it is a long trip from Amsterdam to London. Can't you fly home from Amsterdam? Open-jaw air seldom costs more, anymore.

The Man in Seat 61 covers this scenario

Booking opens only 3 months ahead for journeys with a UK domestic element, even though Eurostar itself opens for booking up to 6 months ahead. More than 3 months ahead and you'll have to decide whether to wait (and risk the Eurostar fare going up, which it will) or book the UK journey separately using option 3 below, at a higher price than you'd get as part of a through journey at www.eurostar.com. With 3½ months to go, I'd wait. But with 5 months to go, I'd lock in the Eurostar fare and buy the UK part separately

https://www.seat61.com/UKconnections.htm#Option%201

I too would buy Eurostar now while cheap, and buy Leicester to London later when it becomes available.

Note that even though trains from Leicester arrive in and Eurostar trains depart from St. Pancras, you still need to add 45 minutes minimum (1 hour better) to go through immigration (passport control) before boarding the Eurostar. Plus a few minutes to walk between platforms, buy yourself some lunch etc.
Brexit (I know, this forum is full of posts about things which will not be affected by Brexit), this is one of the areas which might be. If UK citizens have to have full passport checks post-Brexit (the same checks North Americans have always had), this could slow things down at St Pancras. So allow even more time.
News article: https://www.bbc.com/news/business-47365583

Yes, as Chris said, book your Eurostar now and leave plenty of time at St Pancras after your train from Leicester arrives.

I am planning to travel from England to Brussels, visit Belgium and
Netherlands, then go from Amsterdam back to England.

You should do it the other way around, as there are direct trains from London to Amsterdam, but Amsterdam to London requires a change in Brussels at the moment.

Another option is to fly back. KLM or flybe to Birmingham, then train back to Leicester.

Thank you, all, so much for these great suggestions! I appreciate the insight.