If you had the power to lock in the price on the turn of the year to 2019, what's the lowest price you would be willing to accept. Some caveats, you can't buy more bitcoin or bet on the price etc, it's purely based on your current holding.
Personally I would take a price of $10k right now, anything lower than that and I think I would gamble. This is mostly based on my own circumstances and what that value would mean to me financially.
I think at this point you should not increase the amount of bitcoin and wait until the end of the year, I think you should latch out to usd whenever there is profit. You can see that the market in recent days is constantly changing and difficult to predict, so in order to successfully trade you need short-term trading. Bitcoin prices are rising sharply in the past few days but are still bearish in long term.