Don't use a hard side bag as your carry-on, use a soft side bag. You are less likely to have a stuffed soft side luggage taken from you and checked at the gate for being oversized.
That said...
Do you really want to spend several weeks in Thailand with nothing more than what you crammed into a carry-on? You may grow to hate that decision very quickly. True lost luggage is a very rare occurrence. Most "lost" luggage is actually just delayed luggage. That's a pain in the ass, but in most cases the luggage will arrive on a later flight and be delivered to you by the airline the next day. That happened to me once on an AA flight from Narita to LAX. American delivered the bag the next day to my house. Besides, if your bag disappears never to be seen again, your travelers insurance will take care of replacing stuff. The airline will also have a compensation policy for vanished bags.
If you are flying Cathay Pacific, you really shouldn't have any worries. They have had a rough couple of years due to Covid and the Chinese government's crazed handling of the Hong Kong takeover, but stellar service has always been their calling card and they likely still strive for that these days. It's what they're known for. You should be able to trust them.