Actually, you should take a look at [1]. It's made in Rust, inspired by LMDB, and supports a cool feature that allows close to anything to be implemented: allocating any page you want to store anything you want. The BTree storage is optional and you can implement whatever storage system you want. When storing a value to disk, you can allocate pages and decide exactly how you plan to store the bytes, allowing you not to store the length of them or to split your data into multiple pages, etc.
[1]: https://pijul.org/posts/2021-02-06-rethinking-sanakirja