It was pretty sudden but I think "Human Error" was both plausible enough in itself and a good-enough lead-in. And sometimes I do prefer a surprise romance to one that is pushed from the start and/or for years, assuming that we will or trying to force us to love a couple can be more annoying (Dax and Worf, to a lesser extent Paris and Torres). Dramatically the pairing did seem just to increase the drama of the finale, especially the urgency of getting home earlier, but that was an OK decision and tactic (a finale can and even should introduce new elements for more drama).
Seven and the Doctor had chemistry but, kind of one episode aside (kind of "Virtuoso"), it was consistently clear the only romantic attraction was one-way from the Doctor. Seven and Kim could have worked but that would have had to be more built up, with Kim depicted pretty differently in the last 3-4 years.