Serviio

I have been using Serviio for years and pay for a full access license, which gives me the option for access restrictions by user account. Does anyone know if there is a way to select a default user for new devices? I want to default new devices to a limited access profile, instead of full access to all content.

Unfortunately there is no current default user for new devices. They get access to everything. It has been suggested/commented before, but Im unsure if its on the todo-list or not to fix it.

If you have content that you don't want every device to access you should deactivate "enable access for new devices", and manually give each device its user and then enable access.

That's what I've been doing. but dynamic IP addresses (DHCP) makes it difficult to ensure each machine keeps the access level they have been granted, at least without using reservations.

It sounds like you have returning PCs, why not just give them a permanent IP?
If its your own machines, having them on static will also make other permissions that are based around IPs easier. If its guests, personally I dont like giving guests access to the same network I have all my machines on. Guests go on the guest network.

I havent verified if Serviio remember Device ID (or Mac) or just go by IP. But if it goes by IP its kinda unwise to allow access based on IP if you use dynamic.

One alternative is to set up a form of "content forwarder", like BubbleUPnP and have that one only access to what you want to share, and have guests connect to that one. No more need to set up manually for each guest. Just recommend an client app (if phone) and give them the password for BubbleUpnP and they do everything. Because then all users connect through one node with its access instead of having their own device getting a individual setup.
Unfortunately a bit of hassle in more setup, but with lack of those kinds of features you need to look at other tools and/or plugins. I used BubbleUPnP mainly for better Chromecast support, to cast from my phone to TVs but it also supports more features.