This datapack will start you out with a rapidly shrinking border.. Within a minute of world creation, you will be trapped within a small border of only 200 blocks. Don't worry too much about that though, as things wont stay so small for long..

Over the course of 2 hours ingame, the border will slowly expand to 2000 blocks in size..
Then the next session of expansions will begin, with an even larger size of expansion.

I view these different expansion sizes as "Layers".

The first layer lasts 2 hours, and expands the world size to 2000 blocks.

The second layer starts from the 2000 blocks that the previous layer built up.. and over the course of 4 hours it will increase the world size by 500 blocks until it eventually reaches 10k blocks.. where the third layer will begin.

The third layer will start where the second left off, and expand out to 50k blocks from the world spawn.
It does this 1000 blocks each expansion, and will take a full 12 hours to do so.

Finally, once this is done, taking a total of 18 hours from world creation, the world border will start to expand by one block every minute, effectively infinitely.
Unless you want to leave the game running for 30-40 years, it won't reach the maximum border size, but it also provides plenty of space to play in.

I built this mostly to see if I could... Honestly only questioned why I built this as I was writing this description.. This doesn't change much long term, as it expands too quickly to truly limit you, but if you want the early game to be a little slower this might be interesting to you.