Firstly, ask which end device someone will be downloading to, as 30gb on a low end is not the same as on a high end.
Also, Watch out for particularly buggy or laggy addons, as they will create an unpleasant experience for anyone playing. Also, I recommend downloading addon sharer as it remove many errors and bugs from the downloading process.
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=973145750&searchtext=addon+share

Firstly, navigate to the addons section of the menu and click on subscribed. From there, click on the tool icon as shown below and select disable all addons.



Note: Enabling addons after this is a must, but any addons you don't use you should do the following.
Click on the select all button, then de-select every adddon you don't need.Then click enable selected.

Look for addons with low sizes. Below will be an example of a good sized addon.
(Size is 2 MB)
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=410645074
This is a bad addon to have in multiplayer.
(652 MB, good addon when paired with other small ones but not with other large ones. a Gaming PC is reccomended for running many of these.)
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2257255110

Most maps without many props should be fine, no more to be said.
Here are some good multiplayer maps (for sandbox, mostly).

https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=104793138
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=815782148
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=105984257

And of course, GM_construct and GM_flatgrass!

You can select any amount of players, just remember to have friends only on.
(oh yeah, also add people before inviting them, this stops randoms from joining or something)

If you completed my steps correctly, you should experience the following!
.

Q: Instructions unclear, {insert here}
A: Install CS:S irl, then if you are still having the problem Install all source games.

Q: Could disabling addons corrupt them?
A: Yes, but only if you have a bad PC. Also, They should fix upon restarting GMOD or downloading said addon again.
WIP