Patient Gamers

Undertale is the obvious suggestion, although it's more deconstructive than Earthbound.

Someone made a flowchart on what to play after finishing the series.

If you're willing to check out non-RPGs, then I highly recommend Star Tropics (which is on the flowchart). The best way to describe it as a cross between MOTHER (setting) and The Legend of Zelda (gameplay). An interesting thing to note is that Star Tropics is a first-party Nintendo game that's never been released in Japan despite being developed there.
Another thing is that there's a point where you'll need to get a password off of the Internet. The game originally came with a letter from the protagonist's uncle that contained the password, which would only be revealed by dipping the letter in water.

Thinking about it, the recommendations are all over the place. It's probably because the Mother franchise resonates with so many people, but for completely different reasons.

So since you said your favourite parts were...

I love the music, quirkiness, style, pacing, simple yet rewarding tactics in boss battles.

I'm going to recommend the Mario RPGs.

The Super Mario RPG remake is very friendly, and the original was commonly up there with FF6, Chrono Trigger and Earthbound as the best RPGs on the SNES.

The Mario & Luigi games are a lot of fun, and the battle system is really involved.

Paper Mario is another quirky series, but only the original and TTYD (getting a remake on the Switch soon) are truly great imo.

Or if you want something non-Mario...

Bug Fables!

It's a sort of spiritual successor to the Paper Mario franchise (before it strayed from its RPG roots), and it actually has the emotional impact of the Mother games. (I think it's up there with Mother 3...just don't skip out on the side quests!)

I forget when I played it, but it was the game of the year for me!

Great suggestions here already, I'll throw a few more on top:

• Omori (similar and often compared to Earthbound but the silly fun stuff gets darker and more serious as the story goes on)
• Super Mario RPG (another Squaresoft JRPG from the era of Chrono Trigger and FFVI, very fun and satisfying turn-based combat mechanics, quirky to the max, Squaresoft really had a blast creating a Mario style world. The remaster just released for the Switch, but if you don't have one then you can easily get a digital port of the original SNES version)

The reason the Mother series is so good is because it’s completely honest and lacking self-conscienceness. A lot of games today make a fatal error: they draw too much attention to the author. This is especially the case with homage or tribute games. They don’t allow you to immerse yourself and have that pure experience because the game is chock full of these little reminders that you’re playing a video game.

This is going to be a bit of a grating answer, but, aside from Undertale which is a partial homage/subversion, the Mother series is very unique (on purpose).

You might like Live a Live if you want a JRPG. Otherwise, The World ends With You is a gamr that overlays a fantasy world over a seemingly 'normal' setting with a very unique art style. There are versions for different platforms but if you decide to play it you absolutely need to have a (3)DS