I replaced the slave just 3 weeks ago with a long throw version. With the old slave, first & reverse were difficult to get into and would often grind even with the clutch to the floor.
Drained the old black gungy brake fluid from it, adjusted the slave once to remove any freeplay & now all gears shift smoothly.
So I would suggest you start with the cheaper choise of upgrades (a long throw slave) to see if that cures your problem.
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