ETFs/Robo ETFs advocate regular investing via RSPs, while others advocate holding off until market crises and going 'all in' . How do we reconcile these two investing strategies? ?

Most investors are complacent, they believe they have the skill to time markets well when in reality they suck at it. This is the case for an overwhelming majority of professional investment managers, let alone retail/individual investors. You may read more at: https://www.autowealth.sg/strategy.php

At AutoWealth, we have the benefit of observing the actions and consequences of thousands and thousands of investors. From our observation, many investors who invest one-off and didn't commit to a regular investment plan tend to panic and submit a withdrawal request during the heat of market corrections. Whilst we do observed some investors having the discipline to do a one-off top-up to take advantage of the market correction, this group is really small in numbers.

I agree. Time in market beats timing the market. Also doing regular investing can ward the classic problem of when it's 'low enough', and on the flip side, 'is it too high'?