I wonder how much of the land is going to become farmable now
As long as you have money, water, sun, heat, and something better than bedrock, I wouldn't call it unrealistic.
On a longer time scale, the thaw absolutely has impacts on many things. Over time the soil WILL develop fertility, if we are around or not at that point idk.
[0] I rode the Trans-Siberian railway from Moscow to Ulan Bator, and it was days of the exact same scenery: birch trees lining the railway, with the occasional breathtaking scene that was just often enough to keep me staring at trees most of the trip.
Furthermore Siberia will not be exempt from climactic instability, meaning unpredictable weather events (limited seasonal regularity) making YoY farming planning extremely difficult.
Not only that, but the thawing creates extremely unstable ground, leading to very irregular subsidence.