Descaling boiler in thermal store system

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Old Ferroli combi boiler (with the combi function no longer used, I believe copper HE), directly charging an approx 170l thermal store. Does everything I want from it - or will when I find time to get all the controls as I want them (individually controlled rads, variable flow temp depending on demand from rads, that sort of thing). Has spare tapping for possible (intended if/when I have the spare cash and time) add ons like WBS, solar panels on roof, and maybe (if they ever get down to affordable prices) a heat pump.
Boiler is getting noisy, as I assume it's now extracted all the minerals from the larger volume of water - and it wasn't exactly new before I started. We're in a soft water area. For good measure, I've found the AAV on the boiler pump to be ... rather temperamental and prone to not closing properly after letting air out.
As I see it there are a number of way to descale the boiler :

Easiest - dose the whole system with descaler, let it do it's thing for a while, then neutralise, and flush the whole system. Doesn't need any dismantling or special equipment, but does mean introducing a whole load of fresh water into the system - complete with dissolved oxygen and minerals. I'm figuring that the descaler will be fairly dilute, but given time and accelerated by heat in the boiler HE it'll do it's job.

Not too hard - disconnect the boiler from the store, rig up a header tank of some sort, and just treat the boiler - letting the boiler pump circulate the water. Doesn't involve much new water added to the system, and with concentrated descaler should work fairly quickly. But it means opening up at least two joints, and rigging up the header tank.
Variation on this, I have an ESBE VTD322 diverter valve in the boiler loop. I have spare brass cap (comes with a replacement cartridge kit) so had considered machining this out and making some sort of arrangement to give me access to two ports - kind of how Mira sell an insert for some of their thermostatic shower valves to allow for flushing of the pipework before fitting the valve insert. Would avoid disturbing any pipework but means a bit of lathe work.

More expensive - get someone in to do it. Things are "a bit tight" at the moment thanks to some unexpected bills (cat + bad teeth + emergency visit to vet = big bill :cry:), and I expect whoever did it would disturb more joints anyway.

The first question is, taking the first option, is there anything available that doesn't need the "flush it all out" step ? I.e., it can be left in permanently without eating the system from the inside out ? I don't really care if it (for example) creates a bit of sludge in the bottom of the store - there's a fair bit of space that's effectively dead water anyway given the location of the lowest tappings.
If there is such an option, then that would be by far the easiest.

Any other suggestions ?
 
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