Are there any good commercial designs which stand out as being particularly “modernized” without going full custom? Or are the trade-offs too numerous?
The KAC SR15 solves the bolt longevity issues, but now it’s bolt and barrel extension interface are not compatible with standard bolts.
The LMT MRP platform solves most of the other problems, but not as much the bolt issues. But similarly, the LMT MRP monolithic upper cannot be compatible with standard rails, because that’s exactly the point: to move way from the old brittle way of connecting the hand guard to the upper receiver.
KAC SR15 will last upwards of 50,000 rounds without needing replacement parts, but externally it’s no more physically durable than a standard AR.
LMT MRP is among the most solid and sturdy AR out there, with virtually no way you’re going to break it if you throw it around, drop it, and otherwise abuse it. But internally, it will only last marginally longer than a standard AR before needing replacement parts to keep running reliably.
The only problem is diminishing returns. This is true in general, of course, but it's especially true with brute force solutions like this. If I remember correctly, Aermet 100 has roughly twice as much tensile strength as Carpenter 158, but costs about 3x for raw material alone, even before you account for how much harder it is to machine - and that's the best price-to-benefit ratio on my list.
(This is probably why Aermet bolts are gradually making their way in via LMT and KAC, but not anything else.)