It could be an option. One argument against it I imagine is people fat fingering a zero, or missing a decimal point or some, so they do $200 instead of $20 or somesuch.

In the meantime, you can get the augmented Steam browser addon, and buy wallet funds from Steam on the website in your web browser. Augmented Steam will give you a custom amount input box. It's one of the "augments" that makes Augmented Steam useful and worthwhile.

If you were extra clever, you could fiddle with the amounts using your browsers developer tools too.

Those are your two options while we wait for the feature to be added, which may be never, in which cases the alternatives still exist. People have requested this for well over a decade, probably about day two of there being wallet funds. Valve doesn't seem inclined to do it. But they haven't crippled things to prevent any amounts outside their fixed amounts either yet, which is why the alternatives work.

https://augmentedsteam.com/

https://chromewebstore.google.com/detail/augmented-steam/dnhpnfgdlenaccegplpojghhmaamnnfp?pli=1

There's versions for Edge, Firefox and probably other browsers too. Not sure what you're running, statistically it's Chrome though (I run Firefox and use augmented Steam there, but I don't use wallet funds hah)

I understand that it has been requested for years and if you keep the issue fresh in their minds maybe they will do something. I also use Firefox but if there is this work around already they know it and see it. If people can do this without any issues from Steam then it just makes sense to implement it on the main platform. Also I am a person who doesn't trust easily so adding some addon and entering my card information is a huge road block.