[Burridge]

The best defence against cheaters is an active moderation/admin team, the size of which depends on the server and number of players. The gamemode should of course have aids to the team, such as warnings if someone is for example going too quickly, or their health is not lowering. An easy to use admin command system is helpful too, and just quality of life additions to help make their job easier. Making a cheater's life difficult is the best course of action, and a strong admin team backed up by good systems that can catch a large majority of them helps. If cheaters see an active admin team and a system that is working against them, they'll go somewhere else.

Summing up my point I will say that you will never truly stop cheaters from messing with your gamemode or system, but that's the same for security in general. You'll never be 100% un-hackable, the idea is to prevent the damage caused when it happens, as well as fixing any loopholes that arise and having as few attack vectors as possible. Changing the packets will happen should a new update be made anyway, as every major sa-mp release does this however, it won't help for long. If sa-mp had more anti cheat tools I also do not think it would help.

cessil made an awesome thread on anti-cheat tips: https://sampforum.blast.hk/showthread.php?tid=220089. It's well worth a read for those who need some pointers. I do not know how effective they are these days, the thread is over 8 years old now, but I am sure some of them are still effective tips.