Sadly, you cannot hesitate and try to figure out who you want to romance in DAI. The romance triggers at a very early point and there is no second chance to re-trigger it (except for Cullen).
For Blackwall, if you want to pursue his romance, you must flirt with him during the battlement cutscene, but that would kinda lock you in his romance as you can no longer pursue others' romances unless you break up with him (and you can never pick that up again). The 'start a romance with Blackwall' doesn't show up until a later cutscene. So it is extremely easy to be ninja-ed by Blackwall. My advise is that you stay out of his first convo in Skyhold till you are sure you want to start a romance with him. His romance locks after his personal quest.
As for Solas, the fade kiss thing does not lock in his romance. You have to complete his personal quest, All New, Faded for her (romancing no one, so you should not flirt with Blackwall during the battlement scene here) in order to start his romance. You have to start his romance during the convo after the quest, if you miss it, then it's over. His romance locks near end game.
For Cullen, he is probably the safest romance option in the game as you can start his romance anytime you want as long as you flirt with him during 1) the first cutscene after arriving at skyhold 2) his chess scene. You can start his romance by simply asking him to talk alone. His romance locks after you complete both wicked eye and Abyss.
I haven't tried Bull and Sera's romances so I can't give you any tips.
The last option available to female elf is Josephine. Her romance is triggered after the completion of her personal quest which is available pretty early. Her romance will eventually lock as you continue her romance arc quests.
Side note: I don't recommend to keep all LI available till the game is nearly complete as you will lose a large amount of romance-specific contents. I suggest you to pursue one before starting the main missions.