If your shower bar is going to be a white, single color, with no design of some sort, you can play around with a lot of different stuff to get more bubbles, since you don't have to worry too much about the soap batter accellerating.
If you have oily skin and want a cleansing bar for the shower, yes, 30% coconut is probably fine. But what I would have done, is to add something really hard to stabilize the lather, get it more shaving foam-like. Add something high in stearic acid, like soy wax or something else. And then, if you want more bubbles (and that shaving foam like lather), I think you definately need to use dual lye.
5% KOH and 95% NAOH, that is a good allrounder in a dual lye recipe. But you need to use a
lye calculator that does dual lye, you can NOT just take out 5% of the NAOH amount and replace it with KOH. Dual lye is easy, the only thing you have to choose is the percentage of each lye, and it will be automatically calculated for you. So it is super easy and will give much more lather (and a better type of lather, I think), especially if you combine it with as high as 30% coconut + 10% castor.
You could add sugar as well (but you might not need it if you go for dual lye). If you don't use dual lye, I would definately add sugar to boost bubbles. Dump it in the water and dissolve it there before you add the lye (it can or will be amber colored, depending on how much you sugar you add), or heat up some water in the microwave and dissolve the sugar in that and blend in with the oils.
If it is a simple, single color soap (goes quickly in the mold), you could also do a water discount. For example use a 35-40% lye solution. If you choose water as percentage of oils, remember that the default value of 38% water is by many considered as too much water. I agree (unless you need much water to give you time to work with advanced designs). I would rather go for 33-35% of oils. But for a single colored soap without design elements, I would use a 40% lye solution. Just have everything ready, so you have time to dump it in the mold before it eventually thickens up before your very eyes. Lard is said to be slow moving, as is olive oil, so it might work just fine. I'm just prepared for disaster all the time, becuse that's what I'm used too. And I didn't know that a strong lye solution could make it worse. But it can. But a strong lye solution and pouring at thick trace will remove or minimalize the problem with soda ash on the top and glycerin rivers inside. So it is not all bad. It is also less water that has to evaporate out when curing.
I would also add kaolin clay. For example a tablespoon per pound of oil (just blend it in the oils). That is not a low amount of clay, but I think it will help improving your lather and give slip and silkiness to your shower soap. Castor can be used in higher amounts if you make sure to have enough hard oils in your mixture, and if you use something to balance out the drag they say castor can give in higher amounts. 10% is considered quite high, but it is not too high. If you use clay together with a higher amount of castor, it will feel very slippery and nice, in my experience.
I would drop the 10% canola oil and replace it with something hard, like soy wax, shea butter or whatever with a high amount of stearic acid (but not use pure stearic acid in cold process soap, it will seize the soap if you use it over a few percent). I would do that for two reasons - improve the lather and give a longer lasting soap for the shower. You could also replace the canola with palm oil (high in palmitic acid, which is very good too). Or just skip the canola and use 10% more lard instead.
If you use dual lye, the soap will be slighly shorter lasting. But if you add more hard oils, it will sort of balance that out again. You could also harden the soap by adding some sodium lactate or replace some of the lye water with vinegar (if you choose vinegar, you will have to do some maths or find a
lye calculator that allows you to add vinegar as an additive, and it will do the maths for you). Salt will also harden soap, but since maximum lather is important for you, salt is not a good choice as it will reduce lather.
If you add sugar, high amount of clay, high amount of castor oil and more hard oils, you probably will not have a recipe you can work with for hours. It may thicken rapidly. But if it should be a single white color, you can just blend the titanium dioxide in with the oils beforehand, add the lye, get trace and pour it straight into the mold. It should be enough time for that unless you use a fragrance that will seize it in seconds.
I don't know what milk or coconut milk will do in soap. I have not really tried, so I have no opinions on that, and I have no clue on how to best add it.
Good luck, I think you will have a wonderful bar of soap for the shower, whatever you choose to do or not
I think I will have to make one too. I have lots of soy wax I must use before it is too late.
