I had some problems when adding new shaders and changing the order of the hdr render-chain...
( I tried to add another bloom layer which uses the first bloom as source image for a second, high-radius bloom blur but with leaving the first bloom as source image for the lensflare )

...looks like this was a bit too much for the beginning

However, I think and I hope that I finally understand it now
I started again from zero and I haven't had any problems this time.

At the moment I keep the bloom and lensflare images seperate to use alpha-blending for the bloom and add-blending for the lensflares with a setting to prevent overexposure.
For this I split sc_hdr.fx into sc_hdr_BloomAndLens.fx and sc_hdr_BloomOnly.fx.

Here is a screenshot which shows the new bloom effect pretty good.

I also added the integer settings.hdr.quality to change the downsampling and bloom quality with
SC_LOW: downsampling with 1 sample per pixel, 1 bloom-stage
SC_MEDIUM: downsampling with 7 samples per pixel, 1 bloom-stage
SC_HIGH: downsampling with 11 samples per pixel, 2 bloom-stages
SC_ULTRA: downsampling with 13 samples per pixel, 2 bloom-stages

Higher quality downsampling will prevent aliasing and makes it possible to use more downsampling and results in smoother blurring.
the bloom quality isn't affected by it, yet but this will be the next thing I'll add

I think I'll also add a video-grain effect, and if it's not too hard an adaption shader.