Repointing before tanking slurry

[Studiobuild]

Hi got Concrete blockwork below ground around 70cm deep Supporting a timber outbuilding I’m making in the garden. It’s letting water in, external stuff is not working, my french drain is pants, dpm has clearly failed. With neighbours garden walls so close and timber structure already built digging around the perimeter is not possible leaving tanking as my only option. There will be a 1” cavity and internal timber studs built on a single course of blocks. I intend to use this cavity as a channel to direct water to a sump pump but would like to try and seal the block work as well. I plan to use sika tanking slurry on blocks as well as the slab but would like to chase out the mortar joints in order to refill with something more watertight first. I think the wall to floor junction is the biggest problem. Any suggestions on what to use, mortar with waterproofing additive? or do I need something else? Any and all advice welcome. I’m getting the impression a tanking slurry will fail under water pressure, perhaps if I Just repoint as mentioned and just focus on pumping out whatever water ingress remains? I only get standing water after heavy prolonged rainfall and as it’s built on a gradient the soil drains out after a day or so.