To be blunt, it sounds like you're complaining because an option you'd like to see explored further is not one most others do (the bolded), and think the support of Cai is entirely for out-of-character reasons (wanting more benefits, wanting Ling Qi to change orientations) and blame the quest/yrsillar for not promoting your preferred option. Note, incidentally that the "squeamish about marriage" bit is actually very in-character with Ling Qi. She's very much not interested in that side of things.
Myself, I started dubious of Cai Renxiang - heck, for rather out-of-character reasons (I didn't think it'd be interesting to follow, and didn't want Ling Qi to be "pinned down"), and went to somewhat in favor of her. Although I'd be most in favor of Ling Qi demurring until she sees the inner sect, to be honest (it's not like there'd be significant changes for either of the parties until towards the end of the second year anyway).
The thing is, Cai Renxiang is probably the most in-character choice for Ling Qi as she is currently. The Cai offer is being encouraged by the person Ling Qi trusts and likes the most (Bai Meizhen), it's made by someone who has been (at least as far as she's aware) honest and above-board with her. Renxiang offered payment for services, gifts for favors done, went along with ideas she had (that moonlit meadow, leasing the big cauldron) and gave her some very useful tutoring in bureaucracy so she could help her mother. She made the offer well in advance - "I'm interested in you, but you can have plenty of time to think about it."
Gu Tai... well, there's of course knowing him less. But also, as I mentioned, she's leery of marriage - and of how noble houses handle it. Rather than being near her family and (likely) at least some of her friends, she'll be in a alien and hostile environment.
The sect... Honestly, I was strongly in favor of this before, and still find it a fairly reasonable choice. The military service would be somewhat more rigid, but Ling Qi would likely continue to be able to do whatever she wants before and some time after. Eventually she'll be slotted in to a noble slot somewhere, but she'll have time on her own.
Imperial service... there's some interest in the equivalent of the Imperial Postal Service; go lots of fun places, deliver important messages, etc etc. But otherwise... not so interesting to me, and Ling Qi hasn't shown any interest at all in direct Imperial service.
To be honest, if after her term of service (or even during the ostensible 'period') Cai Renxiang would probably not be unhappy if Ling Qi continues to actively be a part of the Argent Sect. After all, the Sect is a locus of local power - Ling Qi having influence there is an avenue to influence the Sect in turn (a common tie between the Ducal House and the Sect, adding to Renxiang herself learning there and so forth). And if it helps Ling Qi grow stronger? Well, again, as long as Ling Qi is on her side, that's all for the better.