The official web interface has an optional tweetdeck-style mode which can be used entirely through the keyboard. It's off by default, but you can switch it on in your settings. More info at:
https://fedi.tips/how-to-activate-mastodons-tweetdeck-style-advanced-web-interface/
...and here's how to use the Mastodon website through the keyboard: https://fedi.tips/using-mastodon-through-a-keyboard/