Obviously it will vary from person to person, especially in the case of a reading disability or the Roman alphabet not being your native writing system, etc. I think it's reasonable to not spell out every single case and still suggest that it doesn't make a big difference in reading speed once you have gotten used to seeing it, which happens pretty quickly if it's used a lot.

We don't need to replace the 'ch'. We can replace the 'c' with a 'k' when it makes a 'k' sound, like in cougar or caramel, and with an 's' when it makes an 's' sound, like in century or cilia. Then we can use the 'c' character for when we use 'ch' now.