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I am from Germany. Having said that, I use the following to track down search volume over time:

Searchmetrics Essentials and SEODiver (I hope I am allowed to mention "competitors" here). Both provide great insight on when a certain search query is most popular. And provided that these graphs/diagrams are right, they provide great recommendations on when to focus on which type of products. For the industries I'm preferrably working in, the results are absolutely correct as they're reflected in the inquiries we get from customers.

Addition: SEODiver and Searchmetrics show the volume relative to most popular month and SEODiver also gives the number of searches of the strongest month which provides you with an estimate amount for the others periods.

Neither tool is free, but worth the money.

And obviously, these tools have made me neglect the Google Trends offering - when did they change the layout like that? And how can I "zoom in" to only see a period of 1 year for example?  mixed up Trends and Insights, my bad

While I cannot speak for a whole platform (this will soon be happening), I just tested it with a single page of which I changed the URL structure as well as optimizing on-page - the whole website is one mess of spam.

I can only second the advices the posters before have given you: Wait until the crucial time for you business is over.

I experienced the following: I prepared an optimized text for the adjusted site, figured out the best keyword to rank for (quite competitive)  and decided which URL would serve my needs the best. So the structure before was like www.domain.com/brand_page/_22-key-word2-translatedkeyword.php (ranked 20). This was changed to www.domain.com/key-word.html.

We used 301 Permanent Redirect.

The outcome: After 2 days, the old URL was kicked out of the index. Then it took Google 6 days to index the site with the new URL structure but with disastrous ranking (28 first entry). Then it took around 5 days until we reclaimed rank 20. Three days after, we ranked 10 - and we're still there.

So for the targeted short-tail keyword, it payed off.

BUT: Nearly none of the  long-tail keywords that generated traffic for the old page are ranking keywords for the page right now. Only little by little we start to rank again for them, but it is 3,5 weeks now.

To cut a long story short: I'd do it another time in the year, it is better for the client's business.

Probably not the easiest way:

I'd just run an advanced report with external links to any page on this root domain. You get up to 100.000 backlinks which equal 1/3 in your case.

Then you order them by URL, so you can see which URL provides more than let's say 100 backlinks and with text filter you can extract the exact amount per "mass-linking" domain.

After you identified those sitewide linking sites, you can filter them out, so you have a number of links that's more meaningful.

After I finished work, I will dig through my history and can hopefully deliver.

Update: I spent 1 hour going through my browser history and I was not able to find it. Kinda freaks me out. Sorry.