"For almost two years now, I have been in this crazy push and pull tango with a Sag (surprise?)"After seeing his natal chart AND your synastry, I am not surprised. I will tell you why.
I noticed that he has Venus-Saturn-Uranus conjunction in Sagittarius. This indicates that his affections,love nature,structure,discipline,and his independence,excitement are merged with each other. It can indicate tension,excitement,instability,unpredictability
restriction,limitations,detachment,separation in love as well as the up and down or bipolar relationship or the roller coaster ride that you said. He could find it very hard to express affection because of the instability,inhibitions that he might feel at times. This guy might be very difficult to be in a relationship with.
I noticed that your Saturn-Uranus conjunction in Sagittarius conjuncts his Venus in Sagittarius with an orb less than 1 degre,and that can be very tough. Your structure,discipline AND your independence merge with his affections,love. It can indicate tension,excitement,instability,unpredictability
restriction,limitations,detachment,separation in love as well as the up and down or bipolar relationship or the roller coaster ride that you said. He could find it very hard to express affection with you because he might even feel that you have an unstable and restricting effect on him. There can be coldness,distance between you two. This pretty much adds to his own love,affection issues of his Venus-Saturn-Uranus conjunction. It also indicates what you said
That indicates
"the sexual tension still [and will always] remain between us." keyword is tension when it comes to Saturn-Uranus stuff.
His Mars conjuncts your Moon in Scorpio,and that indicates that his actions,assertiveness ,passion merge with your emotional nature,feelings,instincts in an intense,passionate,transformational,and even sexual way. This could indicate intense passion,sexual between you two as well as emotionally charged relationship which can also involve impulsiveness,impatience. He could seem a bit brash,aggressive,insensitive at times to you,and you can seem to be too sensitive,emotional at times to him.
That also indicates
"the sexual tension still [and will always] remain between us." The conjunction in Scorpio can be sexual.
You have Mercury square Mars,and that indicates that your communications,thinking are in conflict with your actions,assertiveness,passion. You can be impatient,aggressive in communications.
His Saturn opposes your Mercury and squares your Mars for a t-square, and so his structure and discipline are at odds with your communications,thinking as well as in conflict with your actions,assertiveness,passion that there could be great stress and conflict. This indicates that his restricting and limiting you can aggravate the issues of your Mercury square Mars. This also indicates serious frustration and communication difficulties that can lead to arguments and vice versa as well. There is also potential or violence due your Mars squaring his Saturn. It's a very challenging t-square. Any t-square involving both Mars and Saturn is especially challenging. Your t-square is mutable,and so the challenges also involving being changeable,adaptable,and restlessness. A lot of ups and downs,and this can also indicate that roller coaster ride as well as your statement
"For years now, we've been best friends and everyone refers to our frequent bickers as lover's spats."
This is a very challenging synastry overall...any time a synastry has a t-square,it tends to very challenging including a lot of stress,conflict,and struggles. There is a major effort needed to maximize potential.
I also would like to see at least 2 of the big 3(Sun,Moon,Ascendant)compatible for good basic compatibility.
Your Sun in Cancer is incompatible with his Sun in Sagittarius AND they are in quincunx aspect indicate stress,tension between your egos,self expressions which require need for adjustment.
Your Moon in Scorpio is incompatible with his Moon in Aquarius
Your Libra Ascendant is incompatible with his Cancer Ascendant.
all of the big 3 are incompatible. That doesn't look good for compatibility
Your Mercury in Gemini and his Mercury in Scorpio are incompatible AND they are in quincunx aspect indicate stress and tension between your communications,thinking which require need for adjustment
His freedom-loving,independent Sun-Moon blend could be a problem for your emotional,security-oriented Sun-Moon blend
This can easily turn into a love and hate relationship if you are not careful.
The relationship can work if you both want it badly and work very hard to make it work. If you don't,you will end up breaking up on unfriendly terms.
Free will is the ultimate factor.