The latest version 2.0.4-11112 of Synology Drive Client available at the release of macOS 11 aka Big Sur contains menu bar icons which are pretty bulky and are not in-line with macOS 11 minimalistic design.
These alternative icons try to be as close as possible to the macOS 11 graphical design.
This set includes alternative icons for light and dark macOS appearance. Just place both directories mac-white and mac-black under ~/.SynologyDrive/SynologyDrive.app/Contents/Resources/images/tray. When in place in Synology drive client navigate to Settings > Global settings > Display and tick Use minimalist system tray icon.
To copy files into place either use terminal if familiar with or in Finder navigate to your home directory and press CMD + left Shift + . Then you would be able to see hidden .SynologyDrive folder. After you open it, right click on SynologyDrive application and choose Show Package Contents.