You’re my hero Kevin, the thumbnail issue has been bugging me for ages! Hooray!
As for the 4 tags, the @dtube tag gets added to the video by default so you don’t actually need to type that one in during the upload, as itll be put in automatically. So just put the 4 other tags you want to use in like normal and Dtube will always be the first main tag as soon as you upload, regardless of whether you type it in the tag box or not.