The softbrick is caused by the counting try_sys flag. After it reaches 6 then the bootloader switches slots.
Therefore the next boot tries from booting from the overwritten second slot.
So one solution is to reset those try_sys counters to 0.
And an other solution is to use the very same boot command for both slot, like you did now!
I think it should solve your soft bricking misery. But you will know it anyway. 