Best Blog Platform / Format

Hi, I'm still pretty new to SEO and Blogging.

I wanted to start writing a little more ( smaller blogs ) in hopes it will bring me some more traffic.
The Website I really need it for the most, is a very old website, that has been build with iWeb ( a very old application by apple that is not even supported anymore ( but I like the program and the page so I haven't changed it yet) but I do know from a marketing point of view its necessary soon.
.. anyway I would like to write some blog posts on it and it has an option to add a blog page but with Google and SEO in general changing all the time, I don't now if that even makes sense !?
Because the algorithms of Google nowadays might not even detect any for the important facts on those old formats !?

Then I have an online store with shopify which has the blog post page already build in. But I have head, that since shopify is so loaded with shop owners, that it even makes it harder from an SEO point of view to be found ( ranked higher).
Is that true ?

The last website I want to invest more time in is actually a blog side, but more meant to keep some clients informed not really to get more traffic / clients with SEO, but I want to change that.
The side is build with word press. I'm assuming that is the best platform / format ??