From Cornwall to Brussels ... in a day?

Certainly doable, but will take some time. And you will lose an hour due to the time difference as well. Another option is to take the overnight train from Cornwall to London and then you'll be in Brussels a lot earlier.

https://www.seat61.com/sleeper-to-cornwall.htm

I was planning to do this myself in the summer from Plymouth (a little east of Bodmin) but I was very glad I decided to stop a night in London as two trains from Plymouth to Paddington (including mine) were cancelled and I arrived two hours later than planned after a 3.5 hour journey sitting on the floor between carriages because we had three lots of passengers on one train.

Then of course I had to get across to St Pancras, which is pretty quick but still nonetheless needs accounting for. Trains from the west come into London at Paddington and then you change to get to St Pancras, btw. From your post it sounds like you thought you went direct from Bodmin to St P.

At the time I was travelling, Eurostar recommended checking in 90 minutes ahead of your train time. Because I stayed the night in London and was on the second train of the morning, I could have got away with less. But later in the day it gets a lot busier.

My understanding is that if you buy a ticket from Bodmin to what’s called “London CIV” then your Eurostar connection is protected - but make sure about that, because I don’t exactly know the ins and outs of what’s involved. This tells you more, if you scroll down: https://www.seat61.com/european-travel-from-uk-regions.htm

This is the relevant info from The Man in Seat 61:

A closely guarded secret! There are special fares from most railway stations in Britain to a destination called London International CIV, for use with a Eurostar ticket or Stena Line Rail & Sail ticket to the Netherlands.
They come in two versions, Advance (must be booked in advance, the price varies, it's only valid on the specific train you book) and Euro Open/Euro Saver (flexible, usually valid on any train that day, can be bought on the day of travel, price doesn't vary).
Tickets to London International used to exist from every station in Britain, that's no longer the case, but they still exist from most stations and are well worth knowing about. If you have a railcard, it can be used with these fares.

Now, the day I was travelling was not one of the strike days but there were strikes that week and I’m sure it impacted my travel. That situation is over, I think. But just this week the weather has played havoc with trains in Devon and Cornwall.

So theoretically it’s a great idea - but if your Bodmin to London train is cancelled and/or delayed, make sure you know you what your protected connection means. What Eurostar will do for you? Will they put you on the next train, or would it be a case of the next train that has space? And if you’re really late, would that be the following day?

I don’t know the answers to this, but I’m just alerting you to potential issues.

It is do-able. I don't know where the car rental office is in Bodmin, but you would have to then get a bus or taxi down to Bodmin Parkway station.
That train takes you to London Paddington station, then you cross London to St Pancras for the Eurostar.

If you did that route you would want to book a ticket to London CIV- as that includes your cross London tube, but far more importantly if the train from Cornwall is badly late into Paddington, and you miss your Eurostar then Eurostar have to put you on the next available service (whether that is a later train or next morning). That is the only way to have a protected connection and avoid paying $$$ for a new last minute ticket. Personally I would never risk it without the CIV protection.

The other suggestion I would make is to return the car to Newquay Airport, then fly with Ryanair to London Stansted, or Eastern to London Gatwick. Then find another airline from STN or LGW to Brussels leaving a healthy connection time.
Or to do a flight all the way to Brussels on Ryanair fly Newquay to Manchester or Edinburgh, changing planes at either to Brussels. Ryanair don't do connections so you would need to allow a good connection time, but it's a possibility.

Whether one is "smarter" than the other is a view for you to take.

EDIT- If you look into the fares manuals referenced on Man in Seat 61 you find that (unlike on the East and West Coast Main Lines) on GWR a CIV ticket cannot be purchased in connection with an advance cheap ticket, only with a full fare ticket. So a 'Euro High Saver Single' from Bodmin Parkway to St Pancras is £108.40. Then your Eurostar ticket on top of that.

Flights from Newquay to Manchester are with Loganair, not RyanAir. Are Lingus via Dublin might be another option.

Easyjet also fly seasonally to Manchester (Loganair year round), and Ryanair also fly to Dublin (as well as Aer Lingus), then from Dublin to Brussels.

There are a surprising number of destinations from Newquay.

OP I’d remove your full name from your post.

I've done just this, except I left at 3am and got the bus to Bristol and then to London, and subsequently the Eurostar to Paris. I had a few hours free in London which was lovely and is why I recommend leaving as early as possible. Alternatively you could spend the night before in London.

Don't overlook the strongly enforced 90-minute checkin for security and Immigration paperwork before boarding the Eurostar. The Eurostar includes a free transfer by Belgian Rail to either of the other two Brussels stations, if you need it. Several trains per hour, fewer at night.

And you can pay an extra £10 to extend the ticket to any station in Belgium and you have 24 hours to use that extension. So you can stay overnight in Brussels and move on to Bruges or Antwerp or anywhere else the day after.

Thank you all for your advice, which as always on the Travel Forum is enormously helpful.

I would add that you need to check for road construction. I drove from North of Dartmoor on the motorway/freeway to Penzance at the beginning of October and there was a long stretch of road construction with detours, delays, etc. On one detour I even took the wrong exit on the roundabout ( google maps seemed confused). Thankfully I wasn’t in a rush. But it took a fair bit longer to get to my destination. I experienced many train delays as well due to bad weather but also cancelled trains due to staff shortages. Sometimes a train was late and once I got on it continued to go slowly because of a problem further up- one time they announced they were stuck behind a freight train, another time they said they had to stop for about 10 minutes because there was a swan on the track. If you got hit with road construction and delayed trains, you might miss your flight.
Hopefully that doesn’t sound too paranoid, but it could happen and does seem common.