Onyx Boox is probably the largest alternative vendor that specialises in larger e-readers. I use the Onyx Boox Nova and prefer it to the Likebook Mars (the software is better and build quality is superior).
However, there is a problem in Onyx releasing a device before quickly discontinuing the product. For example, despite the Onyx Boox Note Lite being released late 2018 it is not even listed as a product on the official website. There was also the recent release of the Boox Nova, but the e-reader is not sold officially by Onyx. Only the stylus supported Nova Pro is available.
Other than quickly discontinuing devices there is also the confusing list of e-readers. For example, the non-stylus Note Lite was released as part of the Note line despite having no stylus input note-taking capabilities. In the Note line, there are, now, officially three e-readers. The differences between these e-readers are significant and the user needs to be careful that they don’t order a device that doesn’t meet their needs. I think the Note line only needs two e-readers - a premium model and a more affordable one.