I'm not part of the blank house team, but with that being said....
If you follow blank house on tumblr (or...wherever else, idk their other social media if they have any) they say it's a slow burn romance, at least on the love interests end. So at the end of spring, they'll see you as, at most, a new friend. They've only known you for two weeks so it's makes sense.
You can come right out the gate and crush on one of them from the start(or... all of them? I haven't tried it but it seems like you can do that) but it also gives you the options at the end of spring so it can track whether your feelings changed. I don't know if it locks you into friendship at some point down the line. We'll find out as more of the game comes out, but I don't think it will completely lock you out of those options anytime soon, even if it ever does. I think it'll be similar for the other seasons going forward, until you've locked in yourself (please don't quote me on this, I recommend following their tumblr because they get a lot of asks that answer the trajectory and aims for the visual novel)
As for romancing Jamie... Generally respect his boundaries, spend time with him when you can and don't pick on his friends. But once again, spring is the first season so the most you can do, atm, is become closest with them out of the ROs. I don't know how far you've played the spring demo, but I recommend playing it once through if you haven't already. You'll know you're on the right track if, by the end of spring, the ro you want the most affection with talks with you... I'm trying to keep this as spoiler-free as possible, lol.
As for guides, blank house has a kickstarter going on right now where one of the rewards is a guide. Along with game keys, merch, a lot of stuff. They have mentioned that they generally want players to go with their gut when picking their choices, so I don't believe there's a way to completely lock yourself out of a romance (unless you're actively doing so, like not spending time with them, picking on the others, and generally being distant.)
TL;DR- with the spring demo, love isn't really in the cards yet. It's a foundation, and you'll be able to explore more as blank house release more seasons.
Sorry for the ramble, I hope this helps!