You're not being over cautious OP. I'm in similar position. Im about to put a deposit down on a new build. Its not been possible this weekend due to technical issues on their part. The house i want is just over 500k. Exchange is scheduled a week before the autumn statement.
I dont want to enter into a legal agreement not knowing my tax liabilities going forward. But I also understand that they'll have to taper/phase in any changes so I'll probably be ok and not be stung twice - stamp duty and house tax. But theres no guarantees and this is a big move for me into a dream home.
When anyone moves house its an expensive event with a lot of upfront costs. Then theres an adjustment period of getting used to new financial obligations/bills. To not be factoring in potential changes would be reckless. Ignore those who say you're overly anxious, they are judging the situation hypothetically because they are not in your shoes right now in real life dealing with it and the long term potential consequences.
You can only deal with facts as you know it. Gather them all together, take advice and views of others and make a fact based decision.
For you, the facts are nothing has been announced or agreed. Nothing is in place that is new that is known to affect you. Big changines are usually phased.
For me completion would be end of next year on my new build and its over 500k, so I'm looking like I'll be caught in middle of it. I'm thinking of not agreeing to exchange until I know the tax liabilities. I don't want to pay the massive stamp duty I'm facing plus an annual house tax.
Good luck OP.