I have a couple more tips you might not have stumbled across.
My favourite is removal of the carriage with it remaining in the centre of the printer.
On each side of the carriage, the two slider bushes are retained by a screw and flat washer. If you remove these, you can pop your finger under the carriage and slide the bushes out (sometimes they take quite a bit of force, but you can't damage anything with finger force).
Once those are out you can work the front of the carriage up and away from the front slider rod. Then it is a matter of gently sliding it forward and off the bush on the rear. Now, the spring that tensions that rear bush has a hair trigger, and will fly quite some distance if you don't get your hand on it before sliding the carriage off.
The bushes are indexed and so can't be assembled incorrectly. I like to give the fine lubricant slots in the bush faces a real good clean, as they seem to gum up badly depending on which lubricant people use.
Assembly is reverse of removal. This way you don't have to remove the slider end caps or disturb the trailing cables (I ruined my first set as the glue holding the cable ends together had disintegrated and so they fell to bits as soon as I pulled them from the contacts).
I've also found that as the trailing cables age, they can de-laminate around where they curl into the contacts on the carriage, and also the bend that forms where they loop out in the printer. They seem very touchy to any form of movement they are not routinely subjected to (like, oh.. say slipping with a screwdriver and belting them with the palm of your fist).
Speaking of lubricant, I'm one of those sticklers for the right stuff. So I have a bottle of 6040-0855 HP synthetic lubricant that I use. It wasn't expensive, and I figure they went to the trouble of making a specific lube, so I might as well use it. At the rate I go through it I figure the bottle will last me 100 years. I also have a syringe of the right white grease for the X-axis gears, but the part number has long gone from that. It's the same stuff I use in the gears of all my laserjets, and is another HP specific goo. Again, it'll outlast me I'm sure. It turns out the synthetic lube (which is water clear when you buy it) slowly yellows with exposure to UV, so keep it in the dark (like ink). My first refill was with ink that had been sitting on my shelf for a year, and I was amazed by how much it faded in the bottle with exposure to UV. I had to drain and refill the carts with new stuff or else all my prints looked like they'd been pegged to the clothes line outside for a year.
Sometimes I've had trouble with recognition of one of the cartridges, ultimately ending in the printer becoming a monochrome model. Upon dissasembly of the carriage, I put the flexible ribbon for the cart contacts under a powerful magnifying glass and spotted where one of the contacts for the offending cartridge had a break right where the copper track joined the gold contact lump. Rather than spring for a re-furbed carriage, there are a couple of e-bay stores in China that stock replacement ribbons for less than half the price of a replacement carriage.
I always give these a good going over anyway as I completely disassemble the carriage for cleaning, but it turns out they are not very difficult to replace.
I like your spot on the DJ500. I worked in an office for years that had an 800ps, and if I could fit it in, I'd get one. Unfortunately I need the 36" wide carriage, but the hole in my office is only just big enough for the 750, so a 42" 800 would be out of the question. Just way too wide.
I've not done a great deal of research on those units, but I certainly enjoy working on the 700's, and even at 300dpi, on coated paper with good ink they do a credible job for images, and a terrific job for CAD (which is their main use here).
Ink is another one. I've taken up re-filling my own carts, and saved a fortune. I pick up new (expired) carts on e-bay when I get the chance, as I find a good refill may only last 3 or 4 fills before the image quality falls off as the print head degrades, and they are only suitable for CAD.
I keep about 3 full sets of carts, a acceptable, good, & best set that I swap between when I want to make something really shine.
I find that the silicone wipers in the service station tend to scratch the print heads if they are allowed to get too dirty. When I refill a cart I have a good look at the head under strong magnification, and that determines which pile it gets assigned to.
One of these 750's lives at my Parents house for Dad to use, so I keep some sets of carts for them also.
I find that by re-filling them myself, I can keep the little green ink level indicator working, and by making sure that they are swapped out when it goes black, rather than when they fail a cartridge check, I don't tend to burn out nozzles, and the carts last more refills.
I have it down now that I can do a set of 4 in about 15 minutes, but if I'm doing a batch I can probably do 3 full sets in half an hour. I do my wifes Officejet 6500a also. It's supposed to use different inks, but it does very well with the same stuff I put in the designjets.
You guys are lucky in that the best ink and accessories all come from the States. Costs me a bomb in postage (relatively), but it's still miles and away cheaper than even expired carts.
If it's of interest, I can detail some of the do's and don'ts I've learned about refilling too.