increase highest upper ratio with gear hub
Decreasing the rear cog is by far the easiest of your options.  On road bikes, chain rings of 34-39 teeth and rear cogs of 11 are common, so you would be within range. The force on the chain is higher for smaller chain rings, not larger ones.  Slipping probably comes from not having the chain tight enough.  You may be able to move the rear axle aft to tighten it.