Several of you have "hit the nail on the head": a key issue that is making this choice difficult is that I am not a professional.
If you want the TL;DR version of the lengthy exposition below, it is this: what a professional considers to be a "toy" might be the better tool for a hobby user ... and I still find myself going back and forth between these options.
Some of the discussion above, as well as in other threads, greatly reminds me of reading through threads in a forum devoted to metal working - a forum that specifically states that it is oriented to the needs of professional machinists, but does welcome hobby machinists so long as they avoid discussions about hobby-grade tools. At this point, I would consider myself an advanced hobby machinist - far more advanced in that hobby than I am in electronics! - and no little part of that advancement is due to participating in that forum. In particular, I learned to recognize and avoid the sort of mistakes that the typical hobby machinist makes - mistakes that cause problems for which the hobbyist can't pin down the cause, because they don't know where to look, or are looking in all the wrong places, or so on. And yet, at the same time, over many years, I became profoundly aware of the different but equally annoying myopia of the typical professional machinist, e.g., an inability to see that it is quite possible to produce extremely precise work even without machinery costing $250K and weighing 8,000 lbs. The priorities and and mindset of the hobbyist and the professional are very different. The typical professional needs to produce only the required precision - and nothing more - as quickly as possible, and tooling is only a means to that end. The typical hobbyist enjoys the journey as much as or more than the destination, and may enjoy the tools for their own sake; time spent working on the tools may be just as enjoyable as finishing the model IC engine. And of course, the hobby machinist is likely not to have room for the 8,000 lb. machine, and such a machine would be overkill for the projects he or she typically pursues. A "mini-lathe" or "mini-mill" are rightly regarded by the professional as toys compared to the 250K 8,000 lb. behemoth - but those "toys" may be the far better choice for the hobby user, and the hobby user might actually be producing superior work - in terms of precision, or in terms of beauty, or so on - because that is what the hobby user is focused on, without being constrained by the demands of the clock.
No doubt this is an imperfect analogy at best, but hopefully it conveys the point. And it can be extended a bit further. Sometimes the hobby machinist does buy professional grade tools ... especially when they can be acquired at a bargain price ... and of course depending on having the space and power requirements needed. And doing so might well mean giving up some features that are convenient and useful for the hobby user (e.g., variable speed spindle) as a trade-off for the greater rigidity and power of the professional tools. The professional replies, "well, that's stupid - just add a VFD and you'll have the variable speed on the professional tool!" Of course, you'll also have to re-wire the garage to get the 240V 30-amp circuit you need, but hey, that's all part of the overhead for a professional ... ! This is why very often hobby machinists pay more for the hobby-grade tool than he or she would have to spend to get a more capable used-but-still-very-good professional tool.
Again, imperfect analogy, but it does help to illustrate why this choice is proving harder than I thought it would be. Dismissing the Rigol as a toy that can't even do simple RF work misses the point. Everything I'm reading suggests that the Rigol would be more than sufficient for my limited needs. Perhaps I would outgrow it - ? But in the meantime, it seems like it might be an easier entry point. Meanwhile, the Siglent seems to be the better tool in several ways (more like the professional machine tool) ... but as I just now read through the "bugs and firmware" thread on the Siglent, it frankly scared me a bit - more so than reading through the equivalent thread for the Rigol. I'm not entirely sure why ... maybe the Siglent's bugs seem more intimidating to me? As an absolute newbie to the DSO world, I am not sure I even understand what some of the bugs amount to.
I am genuinely struggling between the options. If I had more experience, I would have a better idea of which things I might regret the most if I went with this one vs. that one. If the Rigol were significantly cheaper, it would be easier go that direction, assuring myself that even if it isn't quite the machine the Siglent is, I nonetheless made a prudent choice for a "starter" machine for a hobby user. With no experience, and with them being priced the same ... honestly, I really wish I could find a used SDS1104X-E or DS1054Z for a couple of hundred dollars; that might be the best way to gain experience before buying new. But thus far, the only place I'm seeing used units advertised is on eBay, and the prices they are starting at is equal to or often higher than the cost of the exact same unit new.
I also genuinely appreciate all of the comments above. I take those that come from professional EE's (or at least I assume so based on the nature of the comments) to heart, even as I recognize that as a hobbyist, my needs / priorities / approach are of a different nature.
I WILL make a choice, and soon ... but first I'm still wading through some of the lengthier threads to try to get a better feel for how each machine would be to use.