There is nothing wrong with a slow burn (Smallville's slow burn with Lois and Clark is my favorite iteration of the relationship ever) but some of what you've said here really hits me the wrong way.2) Slow down the romance. I'm not saying remove it, because it's still important, but don't rush into it. Looking over at Marvel (MCU, FOX, Sony), I'd say most of the romance stories felt like paint-by-numbers stuff, as in they had to work romance into the story because casuals want to see romance. I like to joke that Hawkeye's wife is the best superhero WAG/HAB, because there's no need to make her a) someone who constantly needs to be saved or b) someone who unreasonably is involved in the action-heavy sequences and c) isn't forced into the story because, by law, the man and woman have to kiss in the movie. Yup, this would probably hurt box office, but I'm not overly concerning myself (yet) with casuals who won't support Superman in the long run. Also, at least that would give some freedom for Superman to travel to exotic locations and spend more time dealing with the superheroic conflicts. Anyway, this is supposed to be a franchise, so use the sequels to fit it in eventually and within a reasonable pacing.
Again, looking to Marvel films here and then trying to graph that onto Superman flat out doesn't work. The reason a lot of the romance in the Marvel films feels paint by number and doesn't seem to emotionally hit (it's often been compared by critics as wanting and lacking specifically compared to the Reeve Superman films) is distinctly because the romance is tacked on like a subplot and not actually given the attention it deserves. You are also dealing with male superheroes where, again, the romance is not central to the story the way it is in Superman.
Hawkeye's wife is not Lois Lane and it's silly to compare her to Lois Lane. I love Linda Cardellini but she wasn't playing an iconic character. Even Pepper Potts, bless her heart, is not Lois Lane. Lois Lane is an icon in her own right and an actual part of the spine of a Superman media property.
Not having romance in a Superman movie doesn't just hurt box office for "casuals"---it craps on part of the heart and soul of the franchise. Again, you need to look at the mistakes of Man of Steel. I'm a fan of Man of Steel but one of the most consistent complaints about Man of Steel was that there was not ENOUGH actual romance in the movie and that was a problem for a lot of people who came into that film looking for it to be more like the Reeve movie, or "Lois and Clark" of Smallville and were frustrated when Lois and Clark barely were allowed to talk in the movie. Amy Adams was awesome in the movie but it seemed beyond clear that WB was trying to act "above" traditional romance with MOS and IT DID NOT WORK. The audience missed it. The audience made it SUPER clear that they missed it. There were literal articles asking "What happened to the romance in Superman?"
A slow burn romance is fine. A love story that develops over the course of a few movies but gets ATTENTION in each film is fine. But, On the contrary, one could also argue that, at this point, Superman has been married for almost 30 years and it's dumb at this point to keep retreading his younger years and origin story and that they should just make a Superman movie where he's already MARRIED and move on from there. But comparing Lois Lane to Hawkeye's wife is a comparison that absolutely does not work. A Superman movie that isn't making good use of Lois is not a good Superman movie. If she doesn't have anything to do it's because the script is bad and not fully integrating Clark Kent into the story and therefore the story has bigger problems to start with.
If we are following cues from Man of Steel, then next Superman movie needs to do exactly the OPPOSITE of this and commit fully to a true love story and put romance front and center the way the Wonder Woman movie did. (And the wonder Woman movie was, of course, following the Superman formula.)
Again, this actually isn't guess work. We KNOW what draws audiences into Superman. The love story is part of it for a significant part of the audience. It should never be tacked on and it should never be ignored or downplayed you just lost half your audience. Get it wrong? (Superman Returns and, depending on who you ask, MOS) And you piss people off. Get it right? And you can redeem your show. (Smallville later seasons.) And literally EVERY Superman media property has already figured this out. This is not rocket science. Not if you recognize and understand that women are not "casual" viewers of Superman media---they are literally half the demo.