One possibility is the right quadratus lumborum.
Fairly common among those who primary do bilateral training. Although it is virtually impossible see with the nake eye, the body often shifts as it performs a given exercise. In other words, both sides are NOT working equally.
It truly is amazing the disparity between what someone thinks is good form and what his or her body is actually doing.
Furthermore, the five months of lay off in which you "sat alot" most likely contributed in more than a few ways.
The QL originates at the iliac crest and inserts at the 12th rib and lumbar spine. Based on the source, the insertion on the lumbar spine can be anywhere from L1-L5, L2-L5, L1-L4, L2-4; the take-home is that it inserts into most of the transverse processes of the L-spine.
Defranco's agile 8 does not address this. It should be noted that it also does not address T-spine mobility, tightness in pec major/minor, lats, VMO, gastrocs, and plantar facia.
So, contrary to what you may believe, you certainly did not try everything. You simply tried the most popular thing. This is understandable. There is a reason, after all, why McDonald's sells more burgers than any other franchise.
The hip flexor stretch you described does not stretch the QL. And, although QL stretches are important, IF you have developed trigger points, stretching alone will not resolve this.
You also mentioned that "...feel it to a lesser degree whenever I lean down and then up with my lower back straight, in the way you would do a RDL." Perhaps it could be that your muscles of the erector spinae (such as the iliocostalis), multifidus, glute max, biceps femoris long head, semimembranosus/tendinosus are being recruited. If you just said to yourself, man, that's alot of muscles, then you're right. With all these other muscles kicking in, the irritated QL is spared. So this may be why you are less symptomatic when leaning down and up in the way you described.
You will need a spherical object such as a tennis ball (they even make softball sized/shaped foam rollers). Just be very careful. As stated before, it inserts at the 12 rib and you are very near the L-spine, which, by the way, should NOT be foam rolled (I'm sure there may be some exceptions but I very much doubt you fall into that category).
A SAFER bet is to find a skilled therapist, have her work on your QL so you develop a better understanding of where it is and how it feels to have trigger points worked on in a safe manner. You can then transfer this awareness into your SMR sessions. This is the course I recommend.
And, as always, do a general or passive warm up prior to SMR.
Follow the SMR with static stretching. Only morons and publicity-hungry whores trying to makes waves will tell you that stretching makes people weak. Gentle to mild static stretches held for 30 seconds or less will not trigger the golgi tendon organ in a performance-debilitating manner.
Get out of that barbell-only mindset. This is where most people tune me out but I'll say it anyway in the hope that you are in the minority. Barbell work is important for strength and size gains. However, many who live by that alone will, sooner or later, suffer the consequences of an imbalanced physique. You are witnessing this first hand. If this doesn't convince you, then I am sorry...for you, that is.
Now, keep in mind that my thoughts on the QL is an initial hypothesis based on the information you provided. Therefore, if it's not the QL, come back to this thread with an update. It's obviously something and we'll figure it out. It is fairly common for even intelligent people to lose focus and objectivity when they have to deal with the pain on a daily basis.