Series or stand alone?

When I began writing books about Matho and his adventures in the 16th century, I never thought they would stretch into a series,  yet here I am writing the fourth book. Matho features in the first books as a subsidiary character, but with an important part to play plotwise. He also meets and makes friends with Harry Wharton in Fair Border Bride and they were the main charactersin Abduction of the Scots Queen. 

Then Harry took a back seat and Matho went on to operate solo north of the Border and eventually Queen's Courier showed how he came to work for the Dowager Queen of Scotland. I haven't thought of a title for the one I'm writing now,  but I have a good 70,000 words down. 

My question is this: should I rebrand them all with new but linked covers and connect them as a series? Or leave them as stand alone novels? It would mean a fair bit of work, but it can be done over the winter. At this moment in time I am working on the garden pulling weeds like a crazy woman and tomorrow we expect guests from Oz for a few days; not long after that we'll be heading off on holiday.  Nothing will get done about the series until I finish the book I'm working on, but I shall keep thinking about it while all these other things are going on. If you have thoughts on the topic, do let me know!