On my untreated 2" x 3' x 8' wooden pine beds I used raw linseed oil to help with the decay problem. I do not know how much it will help.

I used cardboard in the bottom of my beds. I let it stick out about 1 foot on all sides and then covered the outside with pine bark chips. Bermuda grass is very invasive and I don't know how well just newspaper will work.

Next time try the trex deck planks( plastic planks) or cedar, maybe Redwood.

We have double dug our raised beds (by hand, not tiller) and after years of adding amendments, we finally have an outstanding vegie garden. Beds are 6 foot by 30 feet with a few smaller bed sections for herbs, roses, etc.

As we also have woods, we simply cut freshly dead standing oak for firewood and lay extra logs for the sides of the beds. After the logs begin to rot in a few years I stack fresh long poles of oak on top of them with short rebar to hold in place. I also get all the free mulch I could want from the local electric company which even delivers it for us at no charge. You can check and see if your local tree trimmers or electric company will do the same.

Personally I am not a fan of leaving the ground unbroken and covering it like you are doing. It leaves a hard packed barrier below your garden soil and requires a lot more soil be brought in on top to get it deep enough. I have broken up my hard red clay and mixed it very liberally with composted horse manure, humus, and imported top soil. It also helps insure that my plants get plenty of iron from the clay and other trace minerals that 'soilless' mixes lack. Depending on what you grow this seems to be really needed for a lot of my plants else they suffer mineral deficiencies.

I know it's hard work to bust up the soil and amend it heavily rather than just dumping fresh on top, but over the years I see a huge long term difference in the quality of my gardens if I make that extra effort.

Bermuda grass is every gardeners nightmare, kill it with Roundup, till it, dig it, whatever you have to do to get it as far away as possible. What you see on the surface, is also about 6 inches under the soil.