Focus on one pair of characters
A lot of text-based roleplay with romantic or erotic content focuses on each player running one character at a time, even when each player has a whole stable of characters they play intermittently. Likewise, such roleplay often focuses on one-on-one scenes, even in a game with numerous players. It sounds like the two of you wouldn't be interested a story like "our hero sleeps with everyone they meet between adventures", anyway. So I suggest that you have each player character romancing one NPC.
When you focus on developing these two characters, it should be amusing to see how they relate to your own personalities: Without the characters merely being stand-ins for yourselves, you can still emphasize traits you find endearing about each other... or you could "play against type", since a lot of charming heroes in fantastical fiction have traits that would be truly obnoxious in a real-life partner. The focus on two characters also brings us to my second point...
Build up the tension until it's unbearable
This is a factor that people have been considering in text-based roleplay with online acquaintances for decades. Lots of people love a sexy subplot; lots of people also don't want to seem like they're just in it for the adult content when everyone else is really into the fictional world and characters. So it seems "impolite" to push things in that direction too often or too firmly, and the result is a lot like a "will they or won't they" plot in a TV show.
This can work by adapting formulaic stories that are obnoxious in other types of fiction. In TV, film, etc, a character constantly trying to win another's affection often makes that character seem like a jerk. But when roleplaying with an in-character relationship in mind, you both know that the object of their affection secretly reciprocates those feelings. Likewise, this can work with two characters who secretly like each other but are afraid to admit it - as an audience member, you may be yelling "just kiss already!", but while roleplaying, you're building up how that happens in a way that will be satisfying for you both. This kind of thing is eminently compatible with a serious plot - the back and forth between them builds up over the course of the story, until they finally admit their feelings and fall into each other's arms.
This buildup of tension can easily be driven by circumstances rather than characters' personalities. For example, let's say your character and the NPC they have a crush on are planning a heist together. No matter how much they flirt, they're probably not going to act on it for a while. It may be inconvenient to make a move while they're reviewing maps and buying smoke bombs. It could cause a lot of trouble to steal a kiss while they're picking locks or sneaking around silently. And it certainly ruins the mood when they have to escape through the sewers. But they can be flirting the whole time... and the love scene at the end of the adventure should have that much more impact, after nearly getting distracted from the mission because they were both thinking about how cute the other one was.
But there's certainly also a lot of more sophisticated ways to...
Give them a reason that the relationship is inconvenient
Establish a concrete reason that they can't just fool around all the time, one which fits the tone and setting of the RP. The heist plan in my example above is one that works to defer that side of the plot until the end of an adventure, but you can come up with a good reason that love scenes can only be intermittent throughout a longer storyline.
Maybe they're on opposite sides of a conflict, working together to face a greater threat... and their allies accept that, but publicly revealing their love for each other would be scandalous. Maybe a casual relationship, without a proper courtship, is just seen as improper in their society. Maybe it's a comedic matter, where something explodes or a crazed barbarian bursts through the door as soon as they're getting their clothes off.
The pacing of this may be too obvious or monotonous for you, or it may be a bit of a letdown once the characters get intimate and the built-up tension is relieved. In that case, you might want to insert some more pointed conflict into the romance, and...
Introduce a love triangle
Sure, I still think the focus on a single couple is the best way to proceed, but you're not writing an epic of star-crossed lovers. At some point, there's a reason to break things off - it could be a conflict like the ones described above coming to a head, or a the player character might make a bad decision or a bad dice roll that offends or disappoints their NPC love... or they might just be heading out on separate adventures. That's the opportunity to introduce a new NPC with hearts in their eyes, or an existing NPC with newly revealed affection for that PC.
This is a way to "reboot" the flirtation and the buildup of tension, and introduce a new obstacle. The PC may only have eyes for their prior love, while the new NPC does everything they can to win them over. Or plot-based conflicts between the three of them may make the whole affair a risky flirtation. Or they could party up together, while the PC's split affections cause turbulence in their adventuring. If you're not planning on true love winning out every time, it should be a lot more fun to periodically complicate matters this way.
A lot of this advice could be simplified to "do what works in fiction", but it's important to consider how fundamentally different these plot devices are in a collaborative story. The key thing here is, since you two trust each other and are on board for this kind of roleplay, you can look ahead at the results you're aiming for. Ask how you can set up the type of relationship that you find engaging in fiction - and how you can avoid the cliches that annoy you. When you know what that goal is, you can use those plot devices, alongside the narrative and mechanical progression of the game, to find out how you get there.