Big up to movements targeting hip extension and abduction. Not an expert or professional here, but my understanding is that most knee pain athletes typically have weak and or less than optimally functioning hips. Ankle function might also be worth checking, tight calves impairing dorsiflexion, for instance.
It is probably a good idea to be conservative in flexing the knee and espescially under load, but eventually you have to flex and extend it, under load, and ultimately also ride a bike.
One option that might be worth exploring to strengthen the patellar tendon and remedy pain, preferably under the guidance of a professional, are eccentric squats. They come in a variety of guises, and a basic googling will probably land you with incline board eccentric squats. They might be worth trying. But in my experience there are better options. Namely, there is a very cheap device called “Russian belt” or “Tirante musculador ruf” in Spanish that I myself have used with success in rehabilitating my knee injuries. In short, using it to perform eccentric squats will allow you put considerable torque onto the knee joint, quads and the patellar tendon in the phase where the muscle contracts while being lengthened. The physio I worked with recommended this device and movement it and I have been very happy.
The movement I mean is being performed here: https://youtu.be/XyZDvaImgao
I myself have used a very pronouncedly eccentric rhythm : going down in 4-6sec and going up 1sec. Usually did 4x10-15 reps. It is actually a tough movement when done slowly, so not very conservative after we had established that it did not make things worse. Also started riding rather soon.
This movement is sometimes referred to as Spanish squat because it apparently has been quite popular with spanish footballers. There are quite a few resources online. One upside is that the eccentric squat has been shown to have an analgesic effect on the injured tissues, ie rit educes pain, so this might be used to create a window for performing movements that otherwise hurt. I have not done this, though. But the pain relief is good in its own right. It was counter intuitive, but it worked exactly like that with me: eccentrically applying force to the knee helped.
Finally, this is a review that one might also find helpful, as it discusses remedies, timeframes, progress and so on related to patellar injuries. It also mentions eccentric Spanish squats: https://www.researchgate.net/publication/282046406_Patellar_Tendinopathy_Clinical_Diagnosis_Load_Management_and_Advice_for_Challenging_Case_Presentations
Also a short paper specifically on the movement I mentioned: https://www.researchgate.net/publication/336903268_A_biomechanical_investigation_of_a_Spanish_squat_The_effect_of_trunk_inclination_on_quadriceps_activation
Best of luck and be smart.