Meldrew Dave has been using it for more than 10 years; just RO, some time with bicarb and/or the re-mineralisation filter.
I had used with that filter. Regular testing of the steam boiler showed a maximum hardness of 31+mg/l in Aug 21; the fresh water reading was about 18mg/l. Since then, it’s been going south to about 18-19mg/l.
When I installed the drain valve in Jan 22, I saw very tiny traces of scale on the steam boiler. Perhaps, I was delusional at that time.
I changed the filter last month. I am just feeding the RO since then. If you are still concerned, just feed the plain RO, which might impact the taste. Otherwise, add potassium bicarb.
@Rob1’s excel will give you the details. You already have about 25 mg/l of alkalinity. So, GL weighing the bicarb! 😊