You can download the latest version of WP-Juicebox (v1.5.1.4) from the plugin's own support page here.
Here is a direct link to WP-Juicebox v1.5.1.4.
The version history page is here.
With reference to the points you mentioned:
(1) The 'Create Gallery' button was added to the Manage Galleries page in WP-Juicebox v1.5.1.1.
(2) WP-Juicebox v1.5.1.3 fixed a bug whereby the 'Add Juicebox Gallery' button did not display for the Gutenberg editor in WordPress 5.6.
(3) WP-Juicebox v1.5.1.4 fixed a bug whereby the 'Add Gallery' button did not work for the Classic editor (the gallery shortcode was not added to the page/post in the 'Visual' input mode).
With this in mind, if you use WP-Juicebox v1.5.1.4, you should be able to use Gutenberg or the Classic editor (and you'll have the 'Create Gallery' button as a backup if you run into any problems).
The Classic editor is no longer bundled with WordPress but you can download it as a separate plugin from its download page here. It is still maintained by the WordPress team.
Please note that WP-Juicebox comes bundled with Juicebox-Lite (the free version), so don't forget to upgrade the plugin to use you Juicebox-Pro files by following the 'Upgrading to Juicebox-Pro' instructions here.
Please let me know how you get on and if I can be of any further assistance.
Thank you.