Seems like you can, but not clear how easy it is - https://github.com/reHackable/awesome-reMarkable

You can write software for it but reMarkable as a company has been downright adversarial to the open source community and MOST of the tools on that Awesome page are now defunct because reMarkable has obfuscated API endpoints or changed file-standards to prevent third party efforts.

There is quite a lot of hacking possible on the RM1 and RM2 at least. This repo has a nice overview over the many projects: https://github.com/reHackable/awesome-reMarkable

Many of those projects are defunct, remarkable hacking is truly in a dark ace.

I haven't tried it but the PineNote is an attempt to be a hackable alternative: https://pine64.org/devices/pinenote/

But from what I have read software support for the device is very unfinished and not moving that quickly. So while you can hack on it, you will also likely need to.

(EDIT: Some other comments here appear to be suggesting that this is unlikely to come back in stock.)