as someone who fell down the fountain pen rabbit hole, some brown inks that i love include diamine espresso (which comes in a pack of 20 cartridges and is always inked in my everyday carry), dominant industry lungo no. 116, and monteverde moonstone! if you're feeling adventurous and want to experiment with shimmer inks, i also adore diamine dusted truffle, which is a lovely brown grey with silver shimmer. with shimmer inks, though, it's important to write with them often to avoid clogging, and to rotate your pen as you use it to agitate the particles.
Pentel energel sepia 0.5mm is my all time favorite energel! I actually plan on putting the ink cartridge in my Muji pen body because they fit so well, and I love the simplicity of the Muji pens but not their ink. I also want to get back up of the refills!
Fountain pens are definitely a great way to get a lot of color options. There are resources here on RU-vid for ink recommendations if you're interested.
my favourite brown ink that ive been using for years in my fountain pen is Platinum Brun Sepia - its also waterproof so I tend to use with watercolour washes too! Not to mention how pretty the colour is on its own :)) As for my gel pens, I have hopped on the Zebra Sarasa vintage line cartridges and ive been loving them
I love me some brown and orange inks as well. Always good to declutter and love/use the stuff we already have instead of buying new stuff. Having 5 fountain paints is more than enough ✒️ 😂
This is a kind of random pen struggle I personally have been having, but I find that the Uniball One ink in most color variations struggles with the Midori paper. It hasn't been giving me issues in my Hobonichi Weeks, though. Sorry Uniball, but I'm not giving up my Midori! It might be time for me to swap to a Sarasa ink in my Uniball pen like you did; genius idea btw and thank you for sharing it! Love your content!
Uni style has a brown and it, and uni jetstream, are my go-to gel and ballpoints. Also De Aframentis brown, urban sienna and sepia brown are great waterproof inks
I think the travelers co rollerball takes fountain ink cartridges too if you ever want to try that out. My fave brown right now is Rohrer and Klingner Lilly. You have great taste btw