DLNA server, ad-hoc wi-fi, and connection with an Android DAC player

So... Thank you to everyone who helped me by guiding me on different possible step and process!

I succeed to do it and this is how I did it, working on Raspbian with desktop UI and external NTFS HDD :

Step one : Setup your DNLA server. Nothing more, lot's of tutorial around here, can link the one I use, but it's in French. https://doc.ubuntu-fr.org/minidlna

Step two : setting up a WiFi Direct network and a DHCP server. As a I can hardly describe all the things, there is the link I used to set everything up ! Setting up Wifi direct (wifi p2p) and DHCP server

As for now, I can connect to my phone and my tablet to the RPi through WiFi direct and connect to the DNLA server with my music app (Fiio music app, can work as well with Foobar or Proton Player). Everything is good, no latency problem detected for the moment even on 24bit 192kHz FLAC files. The only thing I noticed is that DSD files aren't shown by miniDLNA. I think it just can't read it, but that's not a big deal for me, as I'll put them in the device internal storage.

Your project is to broad to have a detailed answer. But here are some general ideas to point you in the right direction.

You should start with an access point, not using an Ad-Hoc network. It is easier and well known. Look at Setting up a Raspberry Pi as an access point - the easy way and Access point as WiFi router/repeater, optional with bridge. Use the DHCP server from your internet router to manage ip addresses and resources on your local network. It's mostly working out of the box without knowing that it is used.

Using upnp/dlna you usually have three things: a media server that presents the media, a media renderer that is connected to your output devices (music boxes, TV) and a control point that managed the media renderer. A control point is usually an app on a smartphone. All three components communicate on the local network either by wire (recommended for HD TV media streams) or by WiFi (sufficient for audio streams). Using this environment you can use different media renderer on different rooms (multi rooming media) to play e.g. music from the media server and just control them with your smartphone, walking from the kitchen into the bathroom. Raspberry Pis are ideal for this purposes.

Start with installing minidlna as media server on one RasPi. Then setup a media renderer with Howto install DLNA media renderer. You can use the same RasPi but to have your system scalable I would use another RasPi. As control point you can look at bubbleUPNP. That's a heavy weight and I don't like it because it has much things I don't need. Search for upnp or dlna control point in the app stores. At time I try MediaHouse and upplay.