This is a guide covering the requirements to get all the achievements in Steam. They aren't difficult at all and you'll rarely need to go out of your way to obtain them, so this will mostly serve as an achievement check list since a few of them are hidden.

First Step to Donder
Played Taiko no Tatsujin: Rhythm Festival for the first time.

Drum Beginner
Played Taiko Mode 10 Times

Getting Used to the Drums
Played Taiko Mode 50 Times

I Really Like Drumming Now
Played Taiko Mode 100 Times

No Drums, No Life
Played Taiko Mode 300 Times

Becoming a Daily Habit
Played Taiko Mode on any settings for 3 days in a row.

Pretty self explanatory, just play Taiko mode in Thunder Shrine.

Bring On Any Course
Played Taiko Mode on all difficulties.

You'll have to play Easy, Normal, Hard, Extreme and EX Extreme (press right when selecting Extreme difficulty) at least once each. Clear Success not necessary.

A Little Mysterious
Get a BAD rating in Taiko Mode 500 times.

Hit Those Drums!
Get an OK rating in Taiko Mode 100 times.

Now That's OK!
Get an OK rating in Taiko Mode 500 times.

Oh Really? OK!
Get an OK rating in Taiko Mode 1,000 times.

A Little Cute
Get an OK rating in Taiko Mode 5,000 times.

Pretty GOOD
Get a GOOD rating in Taiko Mode 100 times.

Quite GOOD
Get a GOOD rating in Taiko Mode 500 times.

Too GOOD
Get a GOOD rating in Taiko Mode 1,000 times.

Incredibly GOOD
Get a GOOD rating in Taiko Mode 5,000 times.

Every time you hit the notes you get a rating based on accuracy.

Liking the Drums
Achieved hitting the drums 1,000 times in Taiko Mode.

I Love Beating the Drums
Achieved hitting the drums 2,000 times in Taiko Mode.

Becoming One with the Drums
Achieved hitting the drums 10,000 times in Taiko Mode.

I Know How the Drums Feel
Achieved hitting the drums 100,000 times in Taiko Mode.

Real grind for this one, but I'm pretty sure these count ALL drum hits in any game mode (like toy war, don chan band, online etc.), even if you hit when there's no notes. Aka spam a bit or try harder songs to speed this one up a bit.

Quite a Feat of Lung Power
Finished bursting 100 balloons in Taiko Mode

Balloons are quite common in all difficulties, but if for some reason you wanted to grind for this, Megalovania has a lot of balloons per play.

Don Don! Swing Swing!
Achieved Bursting 100 mallet notes in Taiko Mode.

This one is actually a bit grindy. Mallet notes are found in easy and normal, and very rarely in hard and extreme.
A sort of fast way to obtain this is to go to Select Difficulty -> Play Options -> Special -> Perfect, then play Ao to Natsu (Pop), Zenzenzense (Pop) or Symphony No.5 (Classical) on Easy. They start with a mallet note (with Symphony No. 5 having a second mallet not too long after), and if you miss the note after, it'll count towards the achievement.
Retry, hit mallet, miss note, rinse and repeat until you get the achievement. Note that Spartan mode will not work for this method.

My Level is Moderate
Reached Drum Lvl 10.

My Level is So-So
Reached Drum Lvl 20.

My Level is Up There
Reached Drum Lvl 50.

My Level is Pretty High
Reached Drum Lvl 100.

Remember to open the Rewards Map each time you save up max Taiko Points.

Shopping Beginner
Spent 100 Don Coins.

I'm a Little Thrifty
Spent 2,000 Don Coins.

Big Spender
Spent 10,000 Don Coins.

I Don't Hoard Coins
Spent 20,000 Don Coins.

You get a lot of coins from a lot of things, with about 200 guaranteed coins per song, you'll probably have way over 20k before you hit lvl 100 or 300 plays.

A Little Makeover
Changed clothes 50 times.

You Aren't Alone
Changed Mini Characters 50 times.

Simple, just switch back and forth between 2 outfits and 2 mini characters then exit the Outfits menu, it should pop up after that.

Taiko Master
Unlocked all achievements.

Congratulations, you have finished Taiko no Tatsujin: Rhythm Festival on 100% achievement completion! Even if the achievements are not too cryptic to obtain, I hope this guide helps someone in some capacity. Thanks for reading!

04/12/25 edit: few corrections and tweaks here and there.