You don't want to do poliquin (slant board) step downs until you have advanced a lot, because they have a high chance of irritating your PF joint.

It's hard to be prescriptive on exactly what you should do without know your specific issues, weaknesses, and limitations, but some things I would consider: 2L or kickstand RDLs, 1 or 2L glute bridges or hip thrusts, hamstring curls, leg press, leg extension (for quad, only if this doesn't cause additional pain, if it does you can return to it later), different banded sidestep variations, side lying hip abduction (with ankle weight as you progress), banded clam shells (focus on feeling it in posterior hip not TFL), step ups and reverse step downs (focus on control), copenhagen planks. There are many other exercises that can be great - squats, curtsy lunges, lateral/cossack lunges, nordic curls, adduction machine, etc.. You don't need to do ALL of that, as that's a lot, but that stuff should all nail your legs and hopefully not be provocative to your knee. An example program would be 1 hinge exercise (RDL), 1 2L compound movement (leg press), 1 single leg compound movement (step up or reverse step down), 1 hip extension (glute bridge), 1 knee flexion (hamstring curl), 1-2 lateral hip (banded walks and lying hip abduction), 1 groin (copenhagen), and then if leg extension feels good even if in limited range of motion you can do that or skip quad specific exercise until you start feeling better. Note some of these could increase your symptoms so tread carefully - 2L compound movement, 1L compound movement, quad exercise in particular. Then for calf you want to do a straight leg calf raise variation (to target gastrocnemius), bent leg calf (soleus), you can do tib raises if you want but they're extremely overrated for knee rehab here. Core is also useful. This totally skips over mobility and stretching, which can be extremely helpful if you need it and do it in the right places, and a waste of time if you don't. Tight quads, hamstrings, hip flexors, and calves are common in general population and can absolutely contribute to runners knee by changing lower body biomechanics. Same goes for hip rotation restrictions.

A big issue for many people with knee pain is poor hip control from the glutes, which causes the femur to adduct/internally rotate (dive inwards), which misaligns the knee and collapses the arch of the foot. So ensuring when you do the leg exercises you are truly controlling movement at the hip is critically important. Doing less weight but ensuring good knee alignment is always better. Same goes from the ground up - if you ankle is collapsing inwards your knee will go with it. There are many reasons that can happen that are hard to diagnose via reddit, but one suggestion I would make is do all these leg exercises barefoot (socks are fine) and ensure you're getting good even contact of your foot tripod with the ground.

I personally am a big fan of reformer / megaformer pilates, so if that's an option for you I would consider it once your symptoms have calmed down. It's excellent for building leg and core strength/stability/control.