I have been doing this for a couple of months now ,I have a P3 filter in downpipe of roof and made a deep trap with a drain to try and keep bits down plus some sponge in pipe which I clean regularly.I fill two 100 lbutts then transfer to a 1000l IBC tank with a hozelock pump which only takes a few minutes.I have a TDS of 30 off the roof . I put a washable filter in my RO as there is a few micro bits still getting through ,its a 5 micron filter and so far so good ,the TDS has been at 0ppm for ages now .My tapwater tds is around 270- 300 ish so its bound to make my resin last longer.
My IBC had iced up other day so slightly concerned but all is fine now.Its really nice not to see all RO waste going down drain PLUS I use the rainwater to wash car with Hozelock pump, because of the very low TDS car dries perfect after rinsing so great result.Costs a few quid to set up and bit of messing about but worth it for me.