As a sole tester I can offer you some suggestions here. You may already be on this path.
You have your manual standard path through the process: have you documented this? If not, document it first. Tribal knowledge can disappear very quickly when the one person who really understands the thing leaves. It doesn’t matter how you document at this point.
Next, anything you miss you add to the document. I’ve seen some astonishingly complete test case listings that were built by adding in anything that got missed for each release or patch. What will happen is that your basic regression test will grow – sometimes quite quickly.
Your third major focus happens next: work with your QA department to identify a path to automating your regression. Since you have a QA department of 1, there’s a good chance they won’t have bandwidth to automate without a whole team effort (that’s QA and dev working together).
If the third step can’t happen, the fallback is to use as many people as you need to to perform your regression tests. Sooner or later that time investment will convince someone to allocate time to build test automation – but depending on your environment, it’s much more likely to be “later” rather than “sooner”.