Agents do prefer repeat clients, and it's not personal. An investor buying three properties a year is one relationship producing three commissions, and the agent's cost of acquiring that relationship gets spread across all three. A single purchase carries the full cost of finding you, so the fee per deal tends not to move downward for one-off buyers.
Still, careful once-every-two-years buyers are often easier to work with than they think. You've done reading, you'll answer email, and you're not going to write eleven offers and then decide to rent instead. Agents notice that. The way to signal it is to show up with your financing sorted, a written criteria list, and a decision timeline.
On the fee itself, what usually flexes is what's included rather than the percentage. @canton's point about limited scope is the common version. Some agents will also work a flat fee, which on a lower-priced property can come out ahead of a percentage.
The piece worth thinking about separately from cost: on a single purchase you have no second chance to average out a mistake. An investor buying twelve doors can absorb one bad unit. You can't. That argues for paying for someone who has actually represented buyers on your property type and will tell you to walk away, rather than for the cheapest arrangement you can find. Ask any agent you interview to describe a deal they talked a client out of.