Maybe you can post a daytime photo of the tree so we can add suggestions... and a nighttime one when you've done it!
My elderly parents have got a tree (guesstimate 12m high) in their front garden that Dad used to try to put some fairy lights in but he's far too decrepid now so I've done it 2 or 3 times in the past few years using some of the pro extendable fairy light strings I've got, but couldn't do it last year as the kit was out elsewhere. Hoping to be able to find time nearer Christmas to get it done again this year as a surprise for him. It's a Zarges plus long bit of cane with a hook on the end kind of job - certainly nothing I'd do professionally! A cherry picker would be ideal...