Keeping in mind that I'm not an expert in anything, particularly trikes, I think you are capable of determining the best setting for the boom. Even a year ago when I had not pedaled for six months and began getting my legs back in shape, I never had sore legs or knees. Tired, yes. Sore, no. And my pedaling experience says knee soreness results from being in too high of a gear. Downshift to a lower gear and increase your cadence a bit. I'm not saying that will work for you, but it does work for me.
To set the boom length, sit in the seat and put the arch of your feet on the pedals. At the point where one pedal is farthest from you, you should have a slight bend at the knee. Some people recommend putting your heel on the pedal for this measurement. Check that your chain is the proper length for whatever boom position you settle on.
One issue I had finding the correct boom extension was the angle of the seat. Changing the angle meant I had to change the boom extension. I found I like the most reclined position is best for my pedaling style.
I don't know what bent legs are.
Maybe because I never used to stand up to pedal my upright DF bike, I didn't have to work to transition to recumbent tadpole. My first two rides were 13 miles, followed by 25, then 36 and by the fourth week, 42. Any potential soreness dissipated in the hot tub, I guess.