I've started working on our website and I've found millions of "Search" URL's which I don't think should be getting crawled & indexed (e.g. .../search/?q=brown&prefn1=brand&prefv1=C.P. COMPANY|AERIN|NIKE|Vintage Playing Cards|BIALETTI|EMMA PAKE|QUILTS OF DENMARK|JOHN ATKINSON|STANCE|ISABEL MARANT ÉTOILE|AMIRI|CLOON KEEN|SAMSONITE|MCQ|DANSE LENTE|GAYNOR|EZCARAY|ARGOSY|BIANCA|CRAFTHOUSE|ETON).
I tried to disallow them on the Robots.txt file, but our Sessions dropped about 10% and our Average Position on Search Console dropped 4-5 positions over 1 week. Looks like over 50 Million URL's have been blocked, and all of them look like all of them are like the example above and aren't getting any traffic to the site.
I've allowed them again, and we're starting to recover. We've been fixing problems with getting the site crawled properly (Sitemaps weren't added correctly, products blocked from spiders on Categories pages, canonical pages being blocked from Crawlers in robots.txt) and I'm thinking Google were doing us a favour and using these pages to crawl the product pages as it was the best/only way of accessing them.
Should I be blocking these "Search" URL's, or is there a better way about going about it??? I can't see any value from these pages except Google using them to crawl the site.