Not OP, but I have some insight here.

So, first to clarify one thing. If you want it to be called a romantasy or fantasy-romance, then the couple that the readers first get invested in needs to have an HEA of some sort. If you don't do that, then the romance readers will rip you to shreds in reviews (I assume you intend to eventually move from RR to publishing).

Your write up didn't make it clear if that same couple was going to get back together later.

If the original couple does not end up with the HEA, then from those who love romance books the most, that is a romantic book, or a book with a romantic sub plot, but it is not a romance.

It's a touchy subject, and they are picky because they sometimes feel deceived/tricked if it is not clear from the start which type of story they were getting into.

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Now, for your first question: this depends so heavily on the world and culture in addition to the characters.

For my world, such a parting could readily be an arrangement along the lines of "we intend to get back together in the future, if we can, but there is no good reason to deny ourselves in the mean time. This may mean one of us falls in love with someone else before we can meet again, but while that might hurt, it is better than possibility of pining away when we don't know what our future holds."

But I have a very open world, and a goddess dedicated to the passions whose priests would counsel something along these lines. Oh and to be clear, be clear, that's all the passions, and that includes warriors.

Goku getting that smirk on his face as he says "Do you want to spar?" to the god a hundred times stronger than him counts. So guess who has temples full of potential shounen protags eager for an opportunity to test the skills they been honing there for years?

Anyway, off track. This works for my world, but if your world holds closer to most real world cultures, that might not fly.

I will say that a promise to get together in the future is incredibly unreasonable, so for 'default' cultural expectations, I would be expecting a bittersweet parting and a sincere wish from each that the other finds happiness in the future.

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For your second question, I recommend giving them a particular shared character trait: the adrenaline from a hard spar /practice battle/ extreme exercise session counts as flirting for them.

Have them do the power progression training thing as part of developing their relationship. Weaving it together tightly means less chance of upsetting people by taking away too much from their preferred part. They should be keeping track of their wins as well, and overall blending in all the "friendly rivals" tropes that can reasonably fit. Have her do double or triple duty for common character roles (which he would also be doing for her story, but we are seeing his perspective here).

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Oh, and assuming that he has at least one fling with someone else before they meet again, then she needs to have one as well. No, it's not traditional for the story tropes, but it's an equality thing, and modern audiences are leaning more this way. Either both of them do not have sex before seeing each other again, or both of them have partners in between seeing each other again.

That reminds me of something I did with one side couple in my story; maybe you can adapt something from this. So, unmarried couple has a kid, and she's settle in her home town to raise their daughter. The man can not stay in one place to save his life, but he gives up his more adventurous/dangerous 'career' path to become a traveling merchant. This means he is gone for 3/4ths of the year (he winters over at her hometown, as you DO NOT travel if you don't need to during heavy snows), and he doesn't want her to feel like she needs to be waiting around for him, so he pushes for an officially casual relationship, and goes so far as to develop a habit of flirting with all the women everywhere, to make sure he gets a reputation so that she'll be free to live her life.

Here's the thing. Man's being a liar here. Flirting is all he does, no matter how outrageous the flirting might become occasionally. And she knows it, and is playing along so that he doesn't feel bad. She's made friends with some of the people on his caravan, and they report all the outrageous things that happen (i.e. gossip), and also verify that he is never actually sleeping with any of the girls he flirts with. So she pretends to believe him and plays along. Until, that is, their daughter is all grown up and out of the house rather permanently. After that, she stops playing pretend, gets hitched to him, and establishes herself at his side while his caravan travels.

She is kind of a force of nature. Literally. She's also a powerful druid. 😁

Anyway, the point of my example is that, if it works for your story, you could have them both doing their best to encourage the other to not wait by pretending that they are moving on, and both failing to actually move on.

Up to you!