In Pycraft, I figured out the Potions-with-custom-effects, the problem being that Bukkit's PotionEffectType doesn't actually implement the keyed interface, but instead has its own .values() call, so needs a whole special handler. The code for creating potion with custom effects looks like:

    await potion_of(
            'night_vision',
            "Gopher's Gruel",
            {
                'type': 'fast_digging',
                'duration': 20 * 60 * 8,  # in ticks, so 20 seconds...
                'amplifier': 50,  # likely excessive
            },
            {
                'type': 'luck',
                'duration': 20 * 60 * 8,  # in ticks, so 20 seconds...
                'amplifier': 1,
            },
            player=player,
            world=world,
        )

Custom effects are converted from the dictionaries to the PotionEffect type and the python code for potion_of just adds each of them in turn using addCustomEffect() for the PotionMeta.