Here I post some of the informations I have collected over time, which a new player may find useful.

Of course the gog version is a good start, but if you want to play multiplayer I strongly would recommend to install one of the community patches. The not only have faster map download, but also fixes the security problems the original games has. The community patch I recommend is the lilum voyager patch and the download link you will find below.

Playing in HD Resolutions
The game doesn’t support resolutions like 1080p in the menu, but you can set it manually. For that you go to your install folder and into BaseEF there you open with the Notepad for the Singleplayer the efconfig.cfg and for the Multiplayer the hmconfig.cfg. In it you edit the following lines to your desktop resolution:
* seta r_customwidth "1920"
* seta r_customheight "1080"
* seta r_mode "1"
* seta com_hunkMegs "256"

Update the game interface to HD
If you want to update the game interface to HD, you need to replace many of the LCARS elements (including characters) with a higher resolution. Luckily a good friend of mine did just that and made it very simple do install. Just download his package and extract it to the BaseEF folder. https://www.moddb.com/games/star-trek-voyager-elite-force/addons/ef1-hd-mod

Installing Maps/Mods
Most maps and mods go into your baseEF folder. I highly recommend that you copy the folder before you make any changes to it. That way you can always go back to your original version.
Some custom maps, models or mods need more RAM than the vanilla version. You can either change it via the console with the command /seta com_hunkmegs x, the X stands for the RAM value, alternative you can change the config file, hmconfig.cfg, with the same command. I can recommend in 2021 a value from 256.

How to play online
There are still some official MasterServer left, so it’st still possible to play via the ingame browser. For a better online performance I recommend that you change in your hmconfig a few values:
* seta cl_maxpackets: "125"
* seta cl_packetdup "1"
* seta com_hunkmegs "256"
* seta com_maxfps "125"
* seta rate: "100000"
* seta snaps: "100"

Discord
If you are looking for company or help you should try one of these Discord Servers:
The unofficial Elite Force Discord: https://discord.gg/hqSmErp
Unity One (Star Trek/Gaming Community): https://discord.gg/AzcJxBm

Community Package
Nowadays, it's very hard to play a multiplayer round with just the vanilla version of the game, as most of the server use a custom mod and many different maps. That's why I start a list with the necessary packages, which you just have to copy into your BaseEF folder. Should you know additional download links, please contact me via PM or comment below.

Download: https://www.moddb.com/games/star-trek-voyager-elite-force/addons/map-package-for-ef-classic-mobile-infanterie

Useful Links
Elite Force Wiki: https://www.mobile-infanterie.de
Gamefront (Maps/Mods): http://gamefront.online/files/listing/pub2/Star_Trek_Voyager_Elite_Force
Server List: http://dark-clan.servegame.com/serverlist/EF1/ef1List.php?protocol=24&gamename=1

Lilium Voyager
https://www.mobile-infanterie.de/lilium-voyager/lilium_voyager_1_40_win32_x86_2021-04-15.7z
https://www.mobile-infanterie.de/lilium-voyager/lilium_voyager_1_40_win32_x64_2021-04-15.7z

If I missed some important information, just let me know and I will add it ;)

Highly appreciated - thank you so much!

Two classic Q3 Engine tweaks for those having bad framerates or laggy movement and for the shadows:

cg_shadows 1 is the game default. It shows a blob shadow under the characters, cg_shadows 0 is no shadows. cg_shadows 2 are the rough but more realistic volumetric shadows. It use to be disabled by default because it can result in visual glitches or buggy shadows. Matter of preference. I believe the game looks much better with this.

r_primitives 0 is the default. r_primitives 2 use to help to the framerrate in some configurations with problems of slowdowns, lag, bad framerrates, the result is a very smooth gameplay. Day and night.

Both tweaks are common for every Q3 Engine game

Those variales can be changed ingame via the console. but for a definitive change they can be edited into the efconfig.cfg file into the game folder

are there any recommended custom levels?

Oh, there are way to many custom levels, just check out Gamefront and pick some. Alternative you can also download the package above. It include our favourite multiplayer levels.

So, I made these three changes to efconfig.cfg, load the game... and everything is still 4:3 pillar-boxed. I exit the game, check the config to see if something overwrote it, and the settings for 1920 width and 1080 height are still there, but the game is ignoring these settings. Anything else to try to get the game widescreen?

Also try adding:

seta r_mode "-1"

That was the missing piece, thank you.

Thank you, I missed that part. I will add it now.