Yikes, I go away for a bit and this thread has gone off the rails...
With regard to the Siglent clearance DMM, Siglent played dumb with my query about the calibration status. They said "We do not monitor the status of instruments in the Clearance products" and they offered to calibrate it for a fee. However, I did look up the serial number on their Warranty Status page and inferred that it was manufactured and calibrated the end of November 2023. So the person that bought it should be a happy customer. Side note: it's been reported in other threads that Siglent and their dealers open up equipment before shipment to spot-check or otherwise verify the calibration, even if it's still within the 1 year factory calibration period. Also, Siglent doesn't seem to state calibration specs past 1 year.
I think that the OP should have gone for that. It was a good deal and you get a decently modern UI. Calibration and accuracy shouldn't be such a major concern given the OP's requirements. My opinion is that any time a hobbyist can save some money and use it to buy other TE they should go for it.
As mentioned by Dave, references are typically not adjusted, and this is to avoid disturbing them physically. But from what I've seen, test equipment with closed-case calibration is commonly adjusted, especially if it has a display that you're looking at all day. And it's also common to refer to the adjustment as a part/component of the calibration procedure, and Keysight for example does speak of it that way. Then there is the performance verification terminology that honestly for some equipment could be considered identical to the calibration procedure.
In a metrology context, I can see the point of not making any changes to any of your equipment just to make historical data tracking easier, but for a typical hobbyist or professional it makes more sense to have bench equipment that is adjusted to a reasonably-tight tolerance. If I bother to send in my bench DMM and pay for calibration, I'm going to pay to have it fully adjusted along with the pre and post calibration data so that when it comes back I don't have to incessantly look at the cal data printout while I'm using it. 10V is 10.00000V, and I can just mentally be safe and throw away the last digit without having to refer to the cal data. If I'm REALLY looking at digits, I'll pull out all the cal data for all the equipment involved and scrutinize the conditions. As seen in various bench DMM performance specs, you might have the last THREE digits be off by some amount yet it's still within calibration tolerance. How annoying would that be to use on a daily basis??
For anyone interested in turning into a volt-nut or similar, you will want to dig deep into the metrology board to map your path forward. While not the only path, a common tactic is to purchase older gear that has aged and stabilized. You can have it calibrated & adjusted once and then just self-verify it regularly afterwards. With new gear, you're probably going to have to pay for calibration & adjustment many times as it ages, leading to extra costs.
Spot checking offers some information, but it's so far away from anything proper that it's kind of pointless in this context. The BM857 has over 50 calibration points, for example, covering every function and range. So one photo at 5V is not showing much of anything. If you have a decent bench DMM that is in-calibration, then that can be used to compare against other, lesser DMMs, assuming you can generate the appropriate signals.
For hobby references, the DMMCheckPlus is probably the minimum worth considering, since you can at least check a handful of things, and you can use the precision current source and resistors to tease out a few more values.
Ultimately, it's a pretty big shift if you're going to go down this road.
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