With your budget you will find inadequate hardware, if you are going for an eink Android device. When your eink device is not Android, expect hassle with syncing and not-so-good notetaking, dictionaries, and text-to-audio options.
If you are okay with some hassle, then I for example use a 7.8" screen Pocketbook Inkpad 3 to read mostly PDF files for research. Some say 7.8" screen is not adequate for PDF, but actually in landscape mode it is okay. Pocketbook 3 has pretty good annotation option, less good note-writing options, and syncing notes is a hassle, which is why I actually take notes on computer and smartphone, while reading on the e-reader.
There are enough dictionaries for the device for my purposes, but there are far more dictionaries available for smartphones, so I use the smartphone for dictionaries too. And of course for audio.
The cheapest way to get a Pocketbook Inkpad 3 outside Europe is to buy a Vivlio, which is the same thing rebranded
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