My client has a massive 188 page website that has very poor UX, is not responsive and has very poor conversion. I am planning the new UI design currently and am confident that we can improve the journey and conversion.
He has a very content rich site. In the new site, should I just preserve the sites that get the most traffic currently and set up re-directs for all the rest? Or should I migrate all the content and just adjust the navigation to make it less overwhelming?
All your questions predicate/depend on site's monetization strategy.
For example, if current pages all have great SEO traction in SERPs than any site change best be well thought out + in general the HTML structure should...
1) Only fix any existing HTML errors, as reported by the W3C validator.
Never. Ever. Introduce any new HTML errors or site's SEO can circle the drain.
2) Only lower page weight, so the ratio of cruft (non-content) to content, should reduce. So decreased cruft, which will increase content.
For example, converting a well SEO'ed WordPress site to use a Genesis theme can potentially destroy all your site's SEO traction, because Genesis (at least last time I checked) can produce a site which is 98% cruft + 2% content, due to all the CSS classing + attributes + other non-content junk injected into pages.
When I take on a project for one of my clients like this, first I require them to host with me...
Because wrestling with broken hosting introduces so much noise into analysis, I just refuse to deal with slow + glitchy hosting anymore.
Next I have my client walk through their entire money flow, end to end.
After I understand site monetization strategy, then we work through small increments of change, constantly tracking when GoogleBot visits the site + how page indexing is effected in searches after a 48 hour period.
If it's a hobby site, all this is overkill.
If the site measures profits by the hour or minute, then any change can potentially zero out all income, so in this case best to proceed slowly, with great care.
A redesign is a good opportunity to evaluate the pages of the site and what they are doing. You will need data for this such as recent Google Analytics data. Any page that you decide to get rid of because of low traffic to the page you will want to redirect to the page above it, or a consolidated page that contains similar information. You can also allow the pages to 404 for a few weeks to let Google know for sure that you have rid the site of them. Depending on how high traffic of a site it is, Google may not be crawling the entire site anyway.
With that said, I would not worry about including all 188 pages unless all 188 pages are providing value. A large portion of most websites are doing more harm than good when it comes to SEO because of the low value of the pages. Google looks at that. There is a difference between a site being content rich and content saturated. Hope that makes sense.
A few SEO related things to think about:
Does the page have inbound links?
Does the page have organic traffic?
Does it rank on the first page of Google for any queries?
If you answer yes to any of the above questions you need to think about the cost of removing the page. Yes, you can 301 redirect them to a new page, BUT if you want to preserve the rankings, you should keep the page as similar (from a content perspective) as possible.
If you are making major updates to the site architecture, you should look at Google's recommendations for moving a site with URL changes: https://support.google.com/webmasters/answer/34437
This will also help you keep your rankings.