One thing that can vastly improve a cordless trimmers cutting ability is to use the largest diameter cutting string the cutting head can take, and to also use a string that has some sort of ridges or raised areas along the length of the string, instead of just plain old round string.
Every trimmer I have ever used, whether it be a gas or cordless trimmer, I have made sure I replace whatever sting came with it with .095 diameter string that has the raised ridges along its length. The larger diameter string gives it more momentum for cutting as it spins around, and the ridges act like blades to help cut thru more things than just plain old grass.
As a side note, I just purchased a Ryobi Brushless 18V trimmer, that comes with the ability to change out the trimmer head with other things. I have a Husqvarna gas powered string trimmer that can take different attachments like my leaf sucker with a bag, an eight inch pole saw, a blower, a hedge trimmer and a small cultivator.
I have tried all of these attachments with the new Ryobi trimmer, but a few of them, like the cultivator, don't work really great as the 18V batteries don't have the power to run it very well.
But overall, the new Ryobi trimmer works great for what I bought it for, and that is to weedwack the grass around my property that the lawn mower can't reach when I don't have enough gas for the Husqvarna trimmer. As a reference, my property is 1 acre with a split rail fence all round it, sidewalks, some bushes, a patio, a shed, a garden and 13 trees to trim around, and I can do that with one 6ah battery, or two 4ah batteries, with about half the power left on the second one.
The Ryobi trimmer is almost 6 feet long, so it works great for me as I am 6 feet tall. The handle is adjustable, so you can move it about a foot up or down to fit you. My wife (who is 5 feet, 3 inches tall) can even use this thing if we put the handle down to the lowest spot. And it is a lot lighter than the gas powered one. I would say by almost half as much, which makes it easier to use even with a large battery on the end of it.
Here is the one I just bought. It came with a 4ah battery. I have a few other Ryobi 18V tools, so I have a bunch of batteries to use.
Jim