I have 3 main deciding factor when choosing route order/first route:
1. Recommended route order (of course). From the least spoiler-y(?) to the most spoilery one. Though I learned this lesson not from an otome game but from Fire Emblem Three houses where I chose arguably the most spoilery route first. Which I later figured out for myself that it would have been better that I had done that route last instead, since it'd give more impact/surprise feeling for the plot twists/spoiler for the other, least spoilery route. Now I apply that lesson to otome games as well and always look up route recommendations. Especially if said otome game has a stronger/more mysterious story.
2. There's an Li I'm very interested in that sometimes I'd ignore the recommended route order. I initially started Piofiore cause I reeaaallly liked Dante's design so I followed what I wanted and went for him first even though he's often in the third or second to last one in the recommended route order. Same thing happening with my current Over Requiemz playthrough where I chose Kaize as my first even though he's often second-to-last in the recommended route order because he's the one that caught my attention the most compared to the other lis.
3. Just let my choices in common route guide me lol. I don't really do this anymore, mainly for thanks to what I explained in my first point. And because the last time I did this, I got an li that ended up not really clicking with me, so much that it kind of soured my first impression, and now I've abandoned the whole game for a long while now.