Need help diagnosing lack of power below ~3k RPM

I have a 1999 BMW 523i E39 that feels extremely slow below roughly 3k RPM and would like some ideas on things I can do to diagnose it. I tried towing a 1200kg trailer recently and really noticed the lack in power at low RPM. It barely wanted to accelerate below 3k RPM so I kept it above when getting going but noticed that when I shifted up once I were up to speed it started to decrease in speed again, the car just didn’t have the power to even keep it going it felt like. Another symptom the car has is that it stalls extremely easy, especially with the slightest incline. I’ve taken the habit to give it some gas to stop that from happening but if I don’t it either stalls or drops below 300 RPM before starting to roll. I know that somethings wrong as I’ve driven a friends 523i E39 towing roughly 2000kg without any issues.

I can mention that some of the things I’ve done over the last 1.5 years are cleaning the MAF, replacing the fuel filter, cleaning a gummed up ICV, replaced the camshaft sensor and replaced a leaking intake boot. These things were not done in an attempt to fix this issue specifically though but rather because of other issues. One of the fun ones were when the camshaft position sensor failed at the same time as the ICV completely gummed up resulting in a car that would alternate between 1-2k RPM but didn’t respond to the throttle at all. On top of that the car would stall if the clutch wasn’t released at the peak of the alternating "idle".

I can also mention that the car keeps a steady idle but has bad throttle response, I can’t hear any vacuum leaks but haven’t verified further, there are no other weird sounds etc. I’ve researched this a bit and a few things I plan on checking are vacuum leaks as well as the CCV and DISA-valve. I’ve scanned the car and it hasn’t thrown any codes I can use to pinpoint the issue further. Another thing I’ve read is the vanos seals but can they cause this drastic of a change? They might be bad as I replaced the valve cover gasket which felt as hard as plastic about a year ago and they probably are the same then? I’m also wondering if there are any good ways to verify if the vanos unit is working or not before taking it apart? I haven’t found any good information regarding this yet.

I’m planning on going over the car further in the near future so any tips, help or suggestions on possible solutions, things to try/verify etc are greatly appreciated.

Thanks in advance!