For language resources I would recommend two things.
Speakly.me literally the best language learning app on the planet and it has Estonian as its main language due to it being Estonian. There is a Russian - Estonian version I think.
My favorite language textbook ever is Estonian Textbook: Grammar - Exercises - Conversation though if you prefer directly from Russian there should be plenty of options available in Estonia.
I also have a few recommendations for actually learning the language while in Estonia. When I was there my will wasn't strong enough and I am somewhat disappointed in how little I knew when I left.
First off, it will be tough, but Russian is more difficult in my opinion, and that was easy for you to learn so you should be fine.
Secondly, Estonians are not used to foreigners learning their language so they never slow down their talking. I have Estonian friends, who to this day, will speak fast and every time I tell them to slow down, they speed up the next sentence. Older people are better at slowing down than young people.
Third and final comment, tell them, force them, to speak estonian atleast when you've learned the basics. Younger Estonians(most young people in the world tbf) are impatient as fuck when it comes to speaking like a child with another adult. So be committed. And either volunteer in a kindergarten or an old folks home for most language practice partners.
Enjoy Estonia, it's a lovely country <3