Hi, lovely to have you here

Pawing at buckets is a very normal behaviour and both Toby and India do it in my herd. They're very calm around food in general and specifically in training, but they just kick at buckets when they're eating. Since it's not a problem, I haven't addressed it

However, I think you're saying that your horse does it when expecting a bucket, rather than when eating from it? Again, I'd assess the overall level of excitement to see if it really is a problem that you want to address. If you do, then I would train it this way:
I'd begin by desensitising buckets, so start to have empty buckets around. Present your horse with empty buckets, target empty buckets, ride around empty buckets etc. As your horse begins to see buckets in lots of situations and not just high-excitement feeding time, you should see general bucket excitement fade a little.
The next step would be to throw some treats into a bucket when your horse is standing quietly. Maybe you ask him to stand and stay next to an empty bucket and practice walking away and coming back (as per the foundation lesson 'stay'). You can reward him by throwing feed into the bucket or handfeeding, but again he's learning to wait by a bucket calmly and without pawing. If he does start pawing, ideally just wait until he stops for a moment, then mark and return - it sounds like you've been doing a bit of this already, so just build it up as you go further away, or around a corner etc.
Then, repeat the same exercise carrying an empty bucket, so you're getting him used to you appraoching him with a bucket in your hand, but he'lll still be getting rewarded from your hand or the bucket that he's standing by on the ground. With a little repeteition, your horse will learn to stand quietly as you leave and return carrying a bucket. Then, you can repeat with feed in it.
Those would be the steps I'd take to help change that behaviour. If he does it most when you're out of sight, then practice going behind a jump wing/standard or seomthing first whenre you can still see him and then build up to going out of sight. You can use a mirror or just listen out for when he's standing quietly and return at that moment. I think there's a bit of this training in the standing tied videos where I was working on training the horse to stand quietly while I went out of sight into the feedroom - it's in the daily handling course, go to Standing Tied and scroll down to the last video 'standing tied part 2':
https://club.connectiontraining.com/mod ... ding-tied/