Cheers man,
Yeah, I hear you. 
FWIW: Did some testing again locally with my slightly older custom fivem build and latest fivem version with 2 players.
There were some issues with latest fivem with 77731fab-63ca-442c-a67b-abc70f28dfa5 with a changed native’s function signature (used for sms notifications), and then on my older custom fivem build with TaskEscortVehicle function signature on 44febabe-d386-4d18-afbe-5e627f4af937 (adamant). It seemed to be failing because I was using it in a sequence task list, maybe.
Buy yeah, lol I’ve given up on getting it to work with the older script, with the above and other issues. I’m wondering also if it was the old 1604 server that I cant connect to anymore (resource mounter for resource hardcap failed or something) because its no longer supported.
More of a historical note here:
My resource was built with a lot of trail and error as well, and it depended on the fuller entity awareness of clients that I noticed when I was running on the older server (and client I suppose). running with onesync off. In testing both clients would pretty much see all the mission entities spawned by either (even across the whole map away). Vice versa it seemed the mission peds had awareness of both peers (at least within their large AI sight and hearing range settings), and at the very least within 300m of players. Like I would spawn a mission with about 50 peds across the map from both clients, and both clients would see the 50, though you would get cases where with large number of mission peds, one of the clients would not see a few, until they got closer. Smaller missions with less peds iirc were not affected by this, if so only occasionally.
This was the case when running on my old local LAN without other or many custom resources running,.
Saying that, an admin of a highly modded server with a lot of resources used my resource back then for missions, he was using onesync and yeah it was acting like entities only showed up when within 300m or so. I think when he turned one sync off as well, it was the same thing other clients were still not seeing the entities until close up. Which is basically what I am seeing now myself in my environment with the newer ciients and servers now with the older manifest still being used and one sync off.
So long story short, the whole resource would need to be re-written and missions designed to accommodate the apparent lack of entity awareness of clients regardless of onesync status, as well as other issues.
Anyway at this point, any other decent mission systems out there? rhetorical question.
Seeing some potential out there, albeit some are charged.