I tried the free trial period and the ark server ran perfectly fine the other night, so I decided to go ahead and make the purchase. However, ever since I've paid the server has been nearly impossible to play on. It stops responding, resulting in fish flying off into the air, me suddenly dying in the water when I was just on land somewhere else, 2 minute delays for some actions to even take effect like, or just simply dropped from the game altogether and steam saying no response from server.  I'm on a gigabit connection and had no issues running my own non-dedicated server with friends or while playing with the trial test. At the moment, the most I've had online is myself and one other, but this happens even when it's just me.  I have 3 mods installed; gps, dino tracker, upgrade station.  Server ping remains low, with the highest spike I believe only being 100 so I don't know what's going on but frankly it's unplayable. (I have restarted the server and my pc with no change in performance)

Generally what you're describing is a delayed server function, or a queued server funciton. This is caused due to excessive functions being sent to the server. Most commonly this is due to a large entity count either due to spawn set too high or a breeding program that has been left unchecked.

I would recommend checking the entity count on your server, comparing it to the realistic values for that server, and to then adjust accordingly.

For reference, a spawn multiplier of 1 is what WildCard had originally determined to be enough spawns to support thier official servers with 120 active players. That figure was determined when there was only The Island. And towards the end of the game's Official Server life cycle had adjusted their servers to run at 0.85 to 1.05 depending on the map. This setting is generalyl simplified to be figured as 100% for 1, and 85% for 0.85.

So if you were to have this number set to 2 for instance, you would be spawning in 200% of what WildCard presumed initially would support 120 people. That setting determines how frequently and how many dino NPCs spawn.

On non-dedicated, this wouldn't be a problem as the spawning system is tied to the primary player pawn, and only triggers and implements when that player's area of tether is near the spawners. Those commands become queued, and paced otherwise.
The same thing for breeding on non-deidcated. If the player's tether range is not nearby the breeding or hatching dinos, then the system doesn't process thoe actions.

I would recommend performing common admin expected maintenance and cleanup on your server, and then begin investigating where player bases are located at while monitoring the server FPS. (Functions Per Second, can be found in the cheatplayer display.)

You also mentioned your ping was tolerable, but what about RAM usage and CPU usage? I'm presuming that CPU usage may be in the high range with how you're describing. High CPU usage (near 90-100%) will typically be a symptom of low server FPS due to a clogged function queue. Especially with fish flying off into the air. Those types of incidents happen when the client has moved so far passed what the server has been processing that when it tries to rollback / course correct, it skips a few X, Y, or Z coordinates do to how ARK Pathing works.