In addressing your "slippery" front standard issue...I would not use sandpaper. A piece of automotive gasket cork (the "cork-rubber matrix" type), contact-cemented to the underside of the front standard, can provide perfect "slip/grip" properties to help alleviate your issue.

Also...for a quick and effective solution to the bellows-sag issue - first rack the bellows out as far as the focussing mechanism will allow, then reel this back until the bellows reaches is maximum sag point, at which time you can experiment by gently grabbing the tops of various bellows pleats and drawing them forward to the front standard to see which pleat affords the best overall "anti-sag" property.

You can then glue (contact cement) a strip of material (fabric) to the backside of that same top pleat - and then when dry pull this strip forward to ascertain a good location at the front of this strip on which to glue/sew a "D" ring, which itself can then, when needed to help alleviate sag - be clipped over a round-headed, threaded screw affixed to the middle of the top of the front standard.