so yes, that can definately increase the sensation.  Also there is a taboo factor that can seem to add to the enjoyment.

On the other hand, gals, if they are OK with it, are not into the kind of wild hard fucking that they would otherwise allow with vaginal intercouse, so the guy may feel a bit limited in his actions.

Mostly gals have guys enter them from behind, but other options are for the gal to be on top, and also for gals to lie on their back with their legs up on the guy's shoulder.  The latter often makes for the easiest entry, and allows the gal to play with her clitoris while he fucks her ass.

Adding Greek to the routine may help for a while but, honestly, it will still be a routine.  Far be it from me to tell you what to do but I think once a week every week with the same girl may be part of the reason you are bored. Time to add some new ladies to the mix...

Change it up some. You should initiate doing some things you haven't done before. She will follow your lead.
...and yes, as a previous poster said see some other ladies too.

Lots of providers won't do Greek because they tried it as teens with inexperienced partners and it hurt bad. Greek is all about working to not injure a provider.

I use a powdered latex glove and rinse the powder off the glove. I lubricate the small finger of the glove and use that to start lossening up the women that I am preparing for Greek. I work water based lubricant into the woman's anus with the small finger. Once I have the finger in about three quarters of an inch, I add the next largest finger and continue with two fingers. I continue to use lubricant. I loosen the woman with two fingers until she is loose for two fingers. Once two fingers have the woman loose, I add the third largest finger and form a wedge. I work the wedge into the woman still using plenty of lubricant. Once three fingers are in use, I try to lubricant my condom covered penis, the provider may be able to help. Once the provider is loose to three fingers, I start penetrating her with my covered, lubricated penis. I pull my penis out after a half inch stroke and work lubricant in before starting to patiently and gently stoke my penis into the woman. Once I am fully in, I take longer strokes until the women is completely loose to my penis. Once the woman is loose, I go to faster and deeper strokes, all the time paying attention to how the woman is experincing the act. Don't take focus off the woman, she is giving a precious gift, protect that gift.

Expect to require 30 minutes to get the woman loose enough to start your penis penetrating her anus. If with a provider, schedule a two hour session.

If you perform Greek well, the woman will become door shattering loud with her pleasure expressions. You may stop a second and ask her to bite on a pillow.

Some men like to use hard penis shaped sex toys. I avoid using those because I can't get a sense of how the woman is reacting.