I agree with Joea64 – if your rabbit isn’t spayed, that should be first priority. Not only with it protect her from reproductive disease but the hormones in her system could be causing territorial/nesting urges, which can include digging, etc.
If she is already fixed, great. Now it’s a matter of assessing why she’s acting this way and what can be done to stop it. You’re right, it is possible that she is having trouble being an only bun. Some rabbits do better as singletons than others. If you are not in the market for another rabbit (understandably, for your sanity) then she may need more stimulation. Some other members give their rabbits stuffed animals to see if they want a little friend to groom and take care of.
I have a single rabbit and clicker training has been as great way for us to bond and for me to keep his brain occupied – i.e. away from eating all the baseboards. There are plenty of resources online about how to get started.
You mention lots of toys and an xpen, but definitely try mixing up the toys and the items in her pen space. Every time it is in a different setup is a new opportunity for her to explore and not get too used to the way things are. As a digger, you may want to make her a few diggy boxes. Cardboard box filled with torn up paper or the kind of crinkly strips used for filling gift baskets. Cater her toys and activities to the habits she has so, if she is going to dig, it will be in those areas (and hopefully not your rug).
Because you’re in a rental it may also be worth getting a few cheap throw rugs to put in the areas she is ruining. It is hard to get them to stop but at least she won’t be ruining your deposit any further.