I hope your meeting was productive :)
I think everyone has had one of those moments where they exasperatedly give or threaten a much bigger consequence than was warranted. I think I once banned DS1 from screens for an entire month before calming down and realising a, that was just a punishment for me 
and b, it was unlikely to have much of an effect if I did something for that long - by the end of the month he would have forgotten what he'd even done in the first place!
What I then did was spend some time devising a system of tokens that he could earn for doing various things which I was trying to encourage behaviour wise. The tokens could be exchanged for screen time, rather than allowing free access and then revoking it. I then explained to him that although I had said a month that was not actually fair and I was cross when I said that but this is what we would be doing from now on instead. This system worked really well for us for the next few years, both for encouraging specific behaviours (which is more effective than a general instruction to "behave better") and for managing screen time, which was an issue for us at the time.
I would suggest doing something similar. Either reduce the toy confiscation to a temporary toy suspension (which I saw in a later post you said you'll do anyway) or provide specific, achievable targets in order to earn one back at a time.
Since then I've learned loads more about behaviour management and I think the most important thing I've learned is that rather than increase the severity of a threat/punishment, try moving the target instead. So as I said previously, being more specific about what exactly you want to see change is helpful. TBH with the school issue, you'd need to have that discussion with the school - at his age, it's too much to tell him generally to behave better at school, since he probably won't be able to judge this very well for himself, but you could come up with some specific behaviour targets with them, for example staying in his seat, or following instructions straight away, and then focus on those whichever way you prefer, whether he needs to achieve that objective to get his screen time for the day or if he can achieve it for a full week he gets a small prize at the end, either way. If school are willing to immediately mark the behaviour in the moment with a point/sticker/stamp etc, even better. The other really helpful thing that I learnt was that with consequences it's just as effective if they are small as if they are big. We tend to have this assumption that a bigger consequence will have a bigger impact but apparently that is not true. And the benefit of smaller consequences is that they are more repeatable (e.g. with my example of banning screens for a month, I had effectively just disabled my main consequence and would not have been able to use it for that whole month) but also that they are less likely to kick off a counter-reaction when they are something minor that the child feels mildly inconvenienced by rather than feels is a huge injustice.
If you have made the behaviour aim really clear and either a mild consequence or a reward (including praise) is not working to improve that specific aim, our instinct is often to increase the severity of the consequence but actually this is where the idea of "moving the target" comes in again. So for example if it turns out he really is struggling to sit for the amount of time asked, try reducing the amount of time, adding a movement break, or a wobble cushion, and see if you can find the tipping point where the behaviour starts to improve. Once you're at that point you can stay there for a while to try to strengthen the behaviour you want to see and then slowly stretch it as he gets more used to it, or move on to target something else once sitting is more established.
Some people have mentioned ADHD, my eldest two both have ADHD, only DS2 has really struggled with behaviour at school. "Boisterous and strong-willed" could describe ADHD esp along with the other behaviour, but equally, he could just be 5 and finding the transition a bit much along with the other things going on at home. Give him some time and a bit of support (which it sounds like school are doing everything right, I wish DS2's school was as supportive!) and see how it goes. The only other thing I would suggest for school might be to look at things like a wobble cushion and ear defenders if he is struggling to sit and struggling with sound levels. If the school has them available they may be able to be offered generally to the class so he doesn't feel singled out.
IMO it's fine and important to back up school behaviour reports at home but it's also important that children who are struggling at school don't have that bleed into every aspect of their home life, especially if there is any doubt at all about how much they can actually control the behaviour (and FTR I think there ought to always be a level of doubt here.) I do think it's important for there to be some separation between school and home, some treats/relaxation things etc that are offered unconditionally or earnt by something entirely separate to school.
Overall, it sounds like you have a balanced approach and you'll work this out. The chat I just had with DS2's school said watch out for the last couple of weeks before the Christmas holidays as they ALL lose the plot a bit (but esp any children who were already struggling) but they generally come back a bit fresher and rested in January. So don't tear your hair out if it seems to get worse, this is apparently the time of year for it. Consider giving a vitamin D supplement as it can help their energy/focus levels and help boost them through the darker months.