I would say that you should drop out the G13 when removing the "mit silikat" reservoir, refill maybe 3 times with deionised water, then after dropping that out, refill with prefixed G12evo.
I live in Scotland which tends to be a "soft water" area, so I just flushed my ole 2011 Audi S4 with tap water 3 times and refilled with 50:50 premixed G12evo.
I have been monitoring the ph value in the G13 in my wife's 2015 VW Polo 1.2TSI 110PS and it has been dropping back towards being acidic quite quickly - and it does not have Silicate pouches in its reservoir from initial build, I'm doing the same on my older daughter's 2019 SEAT Leon Cupra
290 and it has G13 and a "mit silikat" reservoir and its ph is staying at the same point as when new, so these pouches seem to be doing their job. I just need to encourage her partner a bit more about changing the reservoir and moving onto G12evo.
One strange or interesting thing that I read was, that VW Group in some write up about G12evo, claimed that you can just add tap water to concentrated G12evo - a product that I've not yet seen on sale as typically VW Group outlets in UK are just selling 50:50 G12evo in 1 litre bottles - but maybe that has changed by now.
But, a well respected VW Group Indie based in UK did make it clear that it is only deionised water that should be added to concentrated G12evo and that it is available via TPS as "mixing water" or a similar name - but for us home car fixers in UK, TPS tends to be beyond our reach.