In order to give everyone an easy reference I suggest using the Keypad. It is very similar to what you posted above with one important difference, the numbering starts at the bottom left of the square.

Use the keyboard 10-Key keypad as the reference for numbering the grid.

I used this system for many years in a WWII flying game and it works well.

We used to report position as follows in text:

POSIT<GRID>KP<#>

In your example, in the OP, the position is AF5149x83x23

Using the Keypad system POSIT/AF5149KP3

The keypad system can be used in a manner that becomes more precise if desired.

The Grid AF5149 is broken into nine squares
|7|8|9|
|4|5|6|
|1|2|3|

The boat is in the 3, actually it is in the top left corner of the three or in the 7 square of the 3 square. Break the three square into the same nine squares and the position report becomes

POSIT/AF5149KP37

Two digit keypad position should be all the precision ever needed.

Sorry, but taking the grid reference order provided by the game as a base ( which was historically used) and then reversing the number order when the maps stops displaying it further than 4 levels is just plain silly. Common sense would be to continue the same order just to avoid confusion. Using the keypad ordering has little benefit over the existing order. I would say it is only because of muscle memory for those having a numerical key pad on their keyboards. That may be the majority of players but it is not a rule that everyone has one. Aside from that, those that are more used to telephone keypads follow the KM grid order anyway.

In discord a common counter argument is given that it doesn't matter, as long as it is agreed among players before the game starts. Well, that last part is true. Agreements made on deviations to conventions trumps always. (no politics in there ;) ) But that only works if you know who enters the game session. If you leave the lobby open for players unknown to you and they enter a different boat then you cannot be sure of their level of understanding. Following conventional grid numbering (as was historical and exampled in the game) is then the most simple and right course of action.

My suggestion was based upon my previous experience in another WWII map based game that also did not label the grids beyond a certain zoom level. it used the traditional top left corner starting point (telephone keypad style)

And EVERYONE found it much easier to use the 10-key pad visual reference versus the more traditional telephone style numbering because there was an easy visual reference sitting right in front of them at all times.

It was a flying game and the pace was, obviously, much faster than a submarine game so anything that saved a few seconds and a few brain cells was important.

It doesn't matter much to me what the SOP is eventually but I do know people make fewer mistakes when presented with easy to understand visual aids.

Long years of real world crew resource management training and practice taught me that.

Good point, if it works, use it.

Not sure if this has been mentioned, but the guys I've been playing with radio us their position by stating " Grid x BL(angle) BR(angle)" of the current grid they're in.
Go to map, draw a line from the bottom left of the grid square at the given angle, do the same for the bottom right. Wherever the lines cross is their position.