Hey Annie,
Casetify offers lots of customization, but these cases have to be replaced every couple of years (We change our phones every couple of years anyway), making these an okay solution. The good thing about these is the customization options. You could get any design printed on the straps or the case. I got a back shell for my cousin's iPhone 6 a couple of years back. Sadly her daughter got her hands on it, and chewed one end of the case a couple of months in.
Also, they did not have the customization options that are available on their website now.
Since you mentioned watch straps and iPhone cases, I would suggest Noreve. These are the go to guys for me. Bellroy also has some good cases with great usability options, but they lack the watch straps. Both of these are great options if looking for something with a premium look & feel. they are almost exclusively in leather.
Case-mate also offers phone cases and watch straps, but i can't vouch for their quality as I have not tried them out yet.
Hope it helps, if you are still looking.