I’m in the same situation and for anything paving related I consult https://www.pavingexpert.com/
I was going to follow the advice from there for wet cement and sand mortar, 3 SOFT/BUILDING sand to 1 cement, plus plasticiser. This is a stronger mix than normal bricklaying mortar. However at the last minute I did a bit more research and have gone for Rompox Easy, same price as the Sika.
https://www.pavedirect.co.uk/romex-rompox-easy-pointing-mortar-neutral-15kg.html
(Applying it this week)
Romex have been doing epoxy jointing for a long time https://www.pavingexpert.com/point_romex01
I found it recommended elsewhere first, then found it on paving expert.
Our window cleaner recommended Marshalls Weatherpoint 365, but I’d already bought the Rompox. He said his had been down a good few years and had survived jet washing.
A squirt of washing up liquid into the mortar makes it a bit smoother.
Actual plasticiser liquid is cheaper than washing up liquid and won’t weaken your mortar.