There are many threads similar to this and it is normally quite apparent what is going wrong, but from what you have said it seems that you are doing most things right. It is impossible to tell us everything, but I presume there is a lot more to it as I wouldn't expect you to be having the problems you are having currently.
If you have not been consistent in punishments or have backed down previously, he will push the boundaries or escalate things further to try to get you to back down because you have had enough/it isn't worth it. If you always stick to your guns he will eventually stop, or try a new tactic.
Alternatively there could be a medical reason for his behaviour, or something you have missed that may help further.
There are a few things I picked up on you are doing that will be not be the sole reason for his behaviour, but hopefully they may help you.
If you give a child a count before punishment, you teach them that until the end of the count they can do what they want. That also goes for future behaviour, they start to understand they can do anything they want because later there will be a count and they only need to stop at that point.
Children need to know why things are the way they are, including rules. It is also really helpful because they can apply those reasons to similar situations in future so you aren't having constant battles.
If your reason for no tablet is that it is bed time, tell him it stays downstairs because it is bed time, we can't play in our sleep or it may wake your siblings etc. From that he will learn you don't be noisy and you don't play at bed time, instead of I can't have my tablet because mummy says so/is mean. When he gets a phone, laptop, new toy, he will understand the same rules will apply. Currently all he knows is that tablets aren't allowed, so you will be having another fall out with him in future.
You also mention about increasing the amount of time for a punishment, but at 6 years old they don't understand the length of time very well. Therefore increasing it wont be much more of a punishment.
Finally reward him when he gets it right and doesn't battle with you, it doesn't have to be much. Let him pick his favourite tea for the next day and tell him how great he is.