Minecraft – Keep the Nether loaded

I am building a railway for minecarts with chests that goes from the overworld to the Nether and then back to the overworld. The goal is to transport lots of cobblestone, ores and loots from a far away optimized mine to my base.

Since a minecart with chest cannot travel through a Nether portal more than once in 15 seconds (I found 30 seconds on the Internet but my tests on Minecraft 1.15.2 show me it's 15 seconds: 14 doesn't work, 16 does), I need a way to delay my Minecart for a few seconds while in the Nether before continuing it's journey.

I made a system that does this. It uses redstone, hoppers and special rails.
It works fine… as long as I stay in the Nether.

The second problem that I faced concerning transport through the Nether is the fact that Nether chunks are unloaded while I am in the overworld.

Typically, when mining, I will put all my stuff in a chest, send it, then go back to mining, so the Nether won't load at all, which means my minecarts will be stuck until I load the Nether… and when I pass through the portal I'll find 10 minecarts bumping each others: an unwanted chaos in my workflow.

The solution to my problem is to use a mechanism that keeps the Nether loaded for a while when a Minecart is about to enter the Nether. I found a video that explains how this works as of 1.15. I understood that when throwing items in a portal, the corresponding world will be loaded. The chunk containing the corresponding portal and its 8 surrounding chunks will be loaded and work as if a player was there. Entities will be processed. My delay system will work because it is located in between the portal chunk and a chunk next to it.

The keep-alive system that I built consists of hoppers and dispensers in both worlds that send itselves an item. When system A (in the overworld) receives the item, it sends it back to system B (in the Nether), and vice-versa. I put a delay of 3-4s to test it, and it works fine.

Supposedly, the system that delays my minecarts is permanently loaded in a way that enables it to function correctly even when I'm not there.

However, it does not work.

When I send minecarts from the overworld and wait more than 15 seconds (the delay of my system), even 1 or 2 minutes, and travel to the location in the overworld where it should arrive, it is not there. Both places (departure and arrival) in the overworld are loaded at the same time since they are not that far from each other and I put my chunk loading limit pretty high, so if the Nether was active, my minecart should make it to the arrival.

When I go to the Nether to see what's happening, I always see the minecart stuck in the delay system for about 1 second and then the system releases it and then it arrives where it should. Therefore it seems to me that my system is not completely activated when I'm in the overworld: the items seem to pass from one hopper to the other (which is the core of the delay mechanic), but the rest of the system (the part that activates/deactives the powered rails) is not activated. As a result, the minecart is delayed, but not released.

Every source of information I find confirms me that this should work, but I must be missing a specific detail that invalidates my system. Any ideas? Thanks for reading, stay safe!