I've been programming for many years too but I'm no expert, it's an occasional hobby for me and it takes me a long time to fix problems that happen sometimes as I update and add features to my server. On rare occasions I've had sleepless nights trying to figure out a way of putting a new idea into real code that works for players or getting past an error code so I'm honestly limited in what I can advise, I think you need more help from others with more experience that can guide you.
You ask about why you're seeing warnings using Open.MP and yet no warnings with the SA-MP server but I really don't know. The problem is that this is not supposed to be difficult for server owners to translate/update to Open.MP and I thought things would be easier so yes it's really discouraging and I almost stopped and quit because I was once in your situation. I had to go through all the warnings/errors one by one and change some code inside a few include files then suddenly it all appeared to work.
In your position I would be inclined to pause any change over to Open.MP when I know I can run my game mode and that things will work. Perhaps the Open.MP team or someone on YouTube will create a proper guide and instructions with answers to all these warnings/errors and why they are happening. We should expect the Open.MP client players (when it gets released) to connect to SA-MP servers (samp-server.exe) so why the urgency to move over to Open.MP?
Have you looked into the guidance on this page and perhaps the information or the Upgrader tool can help?
https://www.open.mp/blog/release-candidate-1
Another extreme solution would be to remove the need to even use those includes, stop your dependency on those includes which are causing problems but I know that's a difficult decision to make depending on how integrated and complex your game mode script is but I know that I lost patience with specific functions/includes and just removed them, swapped them with a similar include or tried something else.
Sorry I can't offer any more help really but I know that just searching for those warnings/errors (from your PasteBin link) in Open.MP's Discord channel will bring back results that match your problems exactly and I can see that others have identical problems so you won't need to ask anyone in their channels - there might be answers there if you can get successfully registered and logged into their Discord, then use the Search function.
You ask about why you're seeing warnings using Open.MP and yet no warnings with the SA-MP server but I really don't know. The problem is that this is not supposed to be difficult for server owners to translate/update to Open.MP and I thought things would be easier so yes it's really discouraging and I almost stopped and quit because I was once in your situation. I had to go through all the warnings/errors one by one and change some code inside a few include files then suddenly it all appeared to work.
In your position I would be inclined to pause any change over to Open.MP when I know I can run my game mode and that things will work. Perhaps the Open.MP team or someone on YouTube will create a proper guide and instructions with answers to all these warnings/errors and why they are happening. We should expect the Open.MP client players (when it gets released) to connect to SA-MP servers (samp-server.exe) so why the urgency to move over to Open.MP?
Have you looked into the guidance on this page and perhaps the information or the Upgrader tool can help?
https://www.open.mp/blog/release-candidate-1
Another extreme solution would be to remove the need to even use those includes, stop your dependency on those includes which are causing problems but I know that's a difficult decision to make depending on how integrated and complex your game mode script is but I know that I lost patience with specific functions/includes and just removed them, swapped them with a similar include or tried something else.
Sorry I can't offer any more help really but I know that just searching for those warnings/errors (from your PasteBin link) in Open.MP's Discord channel will bring back results that match your problems exactly and I can see that others have identical problems so you won't need to ask anyone in their channels - there might be answers there if you can get successfully registered and logged into their Discord, then use the Search function.