For scraping, scrapebox or gscraper are much superior to GSA SER. Remember, SER was built to build link not to be a scraper. The amount of proxies you'd need (forget about public proxies, too many will fail in longterm and you'll miss a LOT of potential targets) would be huge and still compare results that cannot compete with the 'pure' scraping solutions.
Another thing to consider is ressources. When SER is running, I'd focus my server ressources on building links much rather than scraping list. An ideal setup is to have a separated server just for running SER (linkbuilding) and another server just for scraping (listbuilding). With several tools like dropbox, you can auto sync the scraped links into GSA SER so it's continously processing fresh scraped URLs => your verified lists will grow like there's no tomorrow.