Alternative to Headscale?
sounds like you just want wg-easy.
Most of what people consider essential features to scale an organisational VPN you have listed as a detraction, so just keep it simple and hand out wireguard configs.
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Ways to backup your docker volumes ?
No. Any database in the last 20 years supports WAL, this advice is straight out of 2005
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Self Hosted User Magement Tools
Sure, a physical notepad and clipboard and not giving anyone a work device.
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