I personally have always ended up with crabs and snails very early on but that's only because I have always got Live Rock from others tanks and then the CUC is included, not ideal but despite temp, pH and salinity swings I have never lost a crab or snail, I would probably get some in.
As for the results they look fine, pH you are aiming for is 8.2-8.4 ideally, 7.8 would be too low. May look odd to those used to tropicals (but even some of those require a pH of 8+). No need for you to do any water change I'm not convinced you ever want a 20% water change in a marine tank, I used to do 20% on my Malawi tank but that's a different thing entirely, in marines you need stability so less sudden changes the better. If you wanted to do a 20% I would do a twice weekly 10%.
Try and find a tank being broken down and avoid the LFS for rock - cheaper and often come with nice freebies plus people always under estimate how much they have, should cost you about £3-5 a kilo, PM me if you want an idea where to look for it.