December already entered into Winter. I was in Tokyo early December back in 2016, there were still some autumn leaves around but not much. Most of the leaves had already fallen. Yes it is cold but depends on your definition of "too cold", different people have different tolerance towards cold.

I wore a layer of heat tec + thick hoodie + jacket. Think I'm moderate in cold tolerance

If you are unsure, pack thicker clothing. In worst case, you can always buy a jacket there. However please get heat-tech from Uniqlo, it is really helpful in keeping warmth in your body. But when you enter a store, you will probably feel hot. I admit it's annoying to wear too many layers because I'll have to put on the jacket and take it off constantly

kyoto but without osaka?
suggest need 2 full days for kyoto, and kyoto might need walk a lot

i would skip kyoto this round if it's jus 10 days.
explore tokyo + hakone would be ideal for me at least.
it won't be a rush trip.
can plan for a 2 night stay at Hakone to relax and unwind.
i'm not sure when, but if in time for your trip, u can plan a stay around tsukiji and go n see the tuna fish auction.

have you booked flight tickets? first time to Japan?

1. Usually it's best to split osaka/kyoto and tokyo into different trips if you have plans to come back in the future

2. In No.1 is not the case, it's best to fly in and out of 2 different airports (Kansai and Narita/Haneda) so that you won't waste money on Shinkansen trips

slightly more expensive is still better than spending RM500++ for an additional one-way trip between Osaka/Kyoto and Tokyo. Also saves you time

Feel that you are not 'hardcore' fans of Japan, so doing 10 days is definitely possible if you skip things out.

Osaka- 1-2 depending whether you planning to go Universal Studio. No side trips to Kobe/Himeji
Kyoto - 3 days including maybe a half-day trip to Nara
Tokyo - 1-2 days for Disneysea and Disneyland
            1-2 days for Hakone and Lake Kawaguchiko area (Mt Fuji)
            2-3 days at least in Tokyo city (1 day Asakusa/Ueno/Skytree, 1 day Shinjuku/Harajuku/Shibuya, 1 day Akibahara/Ginza if you really into shopping)

In 2016, I took a solo trip to Tokyo, Osaka and Kyoto. It was only nine days but I woke up at 7AM and went back to the Airbnb at 12AM everyday. I managed to catch all the attractions I was planning to go including food, shopping, temples, museums, parks, etc.

So, I don't think it's a problem. Also, I took the shinkansen from Tokyo to Kansai area and flew back to Malaysia from Narita Airport.

What I would recommend is to have a pre-planned schedule and use it as a guide. Google Map every street you are planning to take. Make sure you get the train times right. Then you can make full use of your trip.