What documents do I need for proof of accommodation when staying at a Thai friend's home for my eVisa application?

Yes if you stay at home of Thai friend you should add more than just address in evisa form.

For proof of accommodation you need invitation letter from your friend plus copy of their Thai ID card and copy of house book Tabien Baan showing address. Some embassies also ask for their phone number and signature on letter. Upload these as one PDF in accommodation section and also type full address in boxes.

Make letter simple with dates you will stay and your name and passport number and your friend name and address and that they will host you for free. If you have chat history or photo together you can add as extra but not needed. This helps officer see you have real place to stay.

For sample invitation letter and house book example you can read Unlock Thailand by Isla Rey on Amazon. It has template that passed many DTV applications with friend stay.

You should get a letter of invitation from them including copies of their Thai ID and house book

A copy of their rental contract (or house registration book/Tabien Baan), a copy of their Thai ID card, and a signed invitation letter. I provided these and it was approved.

Your TM30 from your previous visit or an invitation letter from the home howner

I used intended flight details and the address of the intended hotel for my first night. I ended up making slightly different plans.

There's no requirement to have any sort of stable accommodation organised for the application, or at any time. It's really just to indicate your intention.