My only thought is why do you need to get into that specific position? I tried and tried as well for a while then shifted gears to be comfortable and focused on mobility separately. Much better progress in both mobility and seated postures now.
when i am stiff especially in the morning i sometimes resort to just cross-legged. as of the lotus position, the hip rotation stretch could help or leaning your back against a wall while doing it
Stretching is an important factor before and after along with continuous practice so there is not large days in between when practicing also make sure your posture is correct Good luck!
I found that meditating on a meditation bench was easier at first. It is easier on the knees but keeps you aligned. I made mine from a quick internet search. Good luck!
I deal with sciatica almost daily and sometimes the lotus position can be a little rough on my back. Usually I’ll stretch my lower body out before sitting. If the position is still too hard I will do a more relaxed fold of my legs and not so tight. Most importantly though is to make sure you are sitting in a comfortable manor where the spine is straight. I also use a meditation pillow (aka my throw pillow from the couch) and use that to raise my hips into a better position putting less tension on my groin and hamstrings. Gymnast Stretching for about 30minutes a day has helped a lot with these problems and has allowed more flexibility in my feet and knees.