While I cannot provide much of an advice regarding the selection of DMMs, I can at least provide some experience with the SDM3065X, which was my first 6.5 digit DMM... Overall, it is not a bad device, and stability is not bad - unless all three of my DMMs chose to randomly drift by exactly the same amount in the same direction, after nearly three years it appears to be still within the 1 year spec (or probably more the 14 days spec), as the overall drift (compared to a Keithley 6500 and 7510) seems to be about 3 counts at the moment over the entire period (in 10V range). Of course that is based on a sample size of just one, so is statistically mostly irrelevant.
Recalibration is reasonably cheap (at least here) - Welectron says 145€, which is not a lot, in that respect most other DMMs of that class are more expensive. What I do like about it is the 10A range at the front, which is quite handy to do recal of the lab PSUs - the Keithleys only support this using the rear inputs or not at all. Less to like is the boot time, although it is not that bad - but the 6500 boots noticeably faster, plus it makes less noise, so it is the usual go-to for some quick measurement. For the stuff where the 6.5 digits or more really matter this is practically irrelevant anyways, as the device has to warm up for at the very least 15 minutes before this makes any sense (and more is more here...), and if working on some actual project where it is on constantly it is not an issue either - really just for when its all about some quick measurement (to stay sane).
There are a few minor issues with the software - I can't understand why it only starts filling the statistics buffer for histogram/trend chart once you first switched display mode, for example, and not just records data there right away, also the scanner card support is a tad rudimentary (if you have the option), though it could be quite useful with only very few minor software-only changes. But overall it works nicely, and in some cases the extended overrange is useful.
Still I will admit that I probably wouldn't buy another one now that I have the pair of Keithleys, but I am not keen to giving it away either - it is still quite useful to have around.