You should read up on the yield line method. There’s been a few discussions on here which would be helpful. See link to thread below, the OP eventually links to a great worked example that covers your case.

https://www.eng-tips.com/viewthread.cfm?qid=210553

Try this, based on Eurocode method.
P398

Just eyeballing it, it looks like your stiffeners may not do too much. The plate may just bend about the face of the column flange. That's how I'd conservatively design it and eliminate the stiffeners.

If you want to take advantage of some sort of stiffeners you will have to figure out how they work. They will be locally bending the column flange where they attach and may not be stiff enough to create reliable "yield lines" in your base plate.

Unless there is something critical about this situation, or you have so many of these base plates, costs could really add up,
I would try to simplify it.
Looking at the segment of base plate between bolt and stiffener / flange as a cantilevered segment of plate, the load
distribution to each element is proportional to the ratio of the cantilever lengths cubed.

Therefore, load to the flange = Tf / (1 + (d1/d2)^3). The remaining tension load (Ttotal - Tf) would be divided
equally between the two stiffeners. Now that you have the load (Tf or Tstiff.) acting on a cantilevered segment of plate,
you can calculate the plate moment and find the required thickness.