I do the same, here's one with links and images to help.

Settings>Device>Developer Options>Apps from Unknown Sources>On

Install "Downloader" from Amazon App Store.

Use Downloader to go to apkmirror.com to search and download Terrarium TV and then Install.

You can manually install Peacock TV on the FireTV.

In Settings > Developer Options, turn on "Apps from Unknows Sources".

Install Downloader from the Amazon App store.

https://www.amazon.com/AFTVnews-com-Downloader/dp/B01N0BP507/ref=sr_1_1?crid=1NWQ9V922SATM&dchild=1&keywords=downloader+app+for+amazon+fire+stick&qid=1612626415&sprefix=downloader%2Caps%2C275&sr=8-1

With downloader browse to apkmirror.com and install the apk for Peacock TV (Android TV).

https://www.apkmirror.com/apk/peacock-tv-llc/peacock-tv-android-tv/peacock-tv-android-tv-1-4-8-peacocktvgoogle-release/peacock-tv-stream-tv-movies-live-sports-more-android-tv-1-4-8-peacocktvgoogle-android-apk-download/

No idea on the memory capacity. I don't see it on your Amazon links so you'll probably have to check the manuals/guides on each manufacturer's website.

Sideloading APKs is easy. If you're somewhat tech savvy the entire process takes 10-15 minutes.

You'll need the direct URL of the APK you want to install (eg www.whatever.com/app.apk) and the Downloader app and the VLC app from the Amazon App store on your TV. The Downloader app handles both the download and installation processes for you.

• Launch the Amazon app store on your TV.
  
• Search for and install the Downloader and VLC app.
  
• Launch the Downloader app, type in your APK's URL and download it.
  
• Navigate to the download location.
  
• Select and install your APK.
  
• Launch your app and when you find the TV episode or movie link you want to stream, play with VLC.

Edit: Formatting.

Use Downloader and install Acestream. Enter the following in Downloader:

http://bit.ly/2gZymTl

Start Acestream, but don't login. Press the home button on the remote. Start Kodi/SPMC. Links should now work.