First of all, yes I have read the article where you can't change the viewing distance in any other software other than PaperMC.
This is the reason why I'm suggesting this.
I have a server where I played with friends a generic survival server, the server started to lag with Vanilla software alone. With that issue, I changed the server software to Fabric to install performance mods, like Phosphor, Lithium, and Sodium.
With that in hand, I went back on the server if everything is fine and working well, and it did, the server is running better. I noticed something after a few minutes that the render distance or viewing distance seems to be lesser than usual.
I went to the server properties file and I tried to change the server viewing distance to 12 but it gave me an "X" or an error in return, I then asked the Aternos server for some support and a moderator said it cannot be 12, so I tried a lower number and in return gave me an "X". I experimented on what the highest viewing distance it can be and it seems to be 5, the maximum viewing distance.
I was sad to see that this was the maximum viewing distance as average most players in the server plays in a 10 viewing distance. I downloaded performance mods to make the server runs better and to make it that way but it changed everything we could see.
I am suggesting to increase the limit at least 8 or 10, what I am doing is solely based to improve performance and not to run other heavy mods.
(As for me they missed an opportunity here, I also think they'll discard this suggestion.)
Anymore question you have to ask I'll happy to answer them.
If I could've edited the server properties file with an text editor, I could've bypass the limit. But it's impossible to do so in Aternos.
If you want to increase it just to make your world look beautiful. Then you can install a client side mod named bobby. World generation engine is broken after 1.14 update, and with this limit, aternos can be still used in only 2gb ram server.