I would suggest that is all entirely unnecessary, and probably will do more damage than it will good....
My open vented system was installed in 1985-ish. In that time, it is now on it's third heat only boiler, and second pump. The first boiler was a cast-iron battleship, which lasted 30 years, it was onl changed, along with the pump, for gas efficiency - both were running fine. I was not around, when it was replaced, but I assume the system was drained then.
The replacement, condensing heat only boiler, proved an unreliable nightmare and was recently replaced with a Vaillant, which has so far proven to be 100% reliable. I drained the system, flushed it, installed TRV's all round, and took the opportunity to swap the galv header tank, for a plastic one, and add the magnetic filter, advised by Vaillant. As said, I flushed the system, I gave it a really good flush through, end to end, top to bottom - the original water was clear, the flush water was clear, nothing at all came out of it. I have a few times checked the filter, and it have picked up almost nothing. The last time I checked it, a few weeks ago, after 12 months, and there was almost nothing in it.
I'm fairly careful about maintaining things, so this system has always had the recommended inhibitor since new. Over the years, I have never had any blockages, never had any rads not working properly, never had any noticeable debris in the system. I would therefore suggest what you are doing is at best, a waste of your time, and possibly does more damage than it will good...
Each time you drain the system, whilst it is empty, the inside of the radiators will have an abundance of oxygen, and be wet - perfect conditions to form rust. Each time you refill the system with fresh water, the fresh water will be well aerated, which is why it can take many weeks of bleeding it out after refilling, and so yet more opportunity to form rust.
Whilst I applaud the care that you give your heating system, as above I would suggest it is too much, but certainly much better than the care the average owner gives their system - judging by all the pleas for help, we see on here.
