Some cranksets are modular. I'm not sure if this is a standard term, but I do mean that, particularly as regards to the question, the crankarms are manufactured separately from the crank spider. Thus, you can replace the spider. Less commonly, on some modular cranks, you can even replace the spindle for a different type, but that's not relevant here (Quarq isn't this type of modular). Shimano's road cranksets are not modular - the crank arms and spider are forged together and can't be separated without destroying the crank.

And now, we come to power meters. Stages popularized a setup where they bonded strain gauges to the crank arms. I believe SRM, the original portable power meter, located its strain gauges more in the crank spider. This can be retrofitted to Shimano cranks, and probably most cranks (in principle, probably even SRAM cranks if there was actually demand for this). Even if that's wrong, Quarq's strain gauges are definitely in the crank spider. So, if it must be a Quarq, you are replacing at least the crank arms, spider, and bottom bracket.

I realize this is complicated, but there are two things to consider with a bottom bracket. One is the bike's BB shell type. Yours is simple, it's a BSA threaded BB. The other is the spindle size that the BB is designed to accommodate. This is where your problem is. Shimano uses a 24mm spindle, so your bike came with a BB for that. SRAM uses the DUB standard, built around a 28.99mm spindle. You cannot cram that into the Shimano BB on your bike, it would not physically fit, and you'd destroy the bearings if it somehow fit and could actually turn. (Just a side note: SRAM did once use the BB30 spindle, which is 30mm in diameter, and I believe that if they said "DFour" they meant a DFour spider with a BB30 spindle, whereas they would probably say "DFour DUB" to mean that spider paired to a DUB spindle. The BB30 Quarqs should be out of production. If you are getting a DFour on closeout, you'll need a BB that takes a 30mm spindle - i.e. a 30mm BSA BB.)

More on the BB - not to nitpick, but while you do need a BSA DUB BB, you don't have to get it *from SRAM* if you don't want to. There are third party manufacturers who make this standard. By third party, I mean that Shimano is not one of those manufacturers. I'm using that in a similar sense to aftermarket.

I said at least the crank arms and spider plus BB because there is a version called the DFour91 DUB, whose spider has Shimano's bolt circle diameter. (Or the DFour91 was the same way but with a 30mm spindle attached.) You could thus reuse your chainrings, preserving the same front shifting quality. I think many observers would consider Shimano's front shifting to be better than stock SRAM, assuming both systems are set up correctly. It's noticeable but may not be a material difference, although the cost differential could sway you towards getting the DFour91 DUB.

To my knowledge, SRAM offers a set of carbon arms branded to Quarq. I'm not sure if the construction is comparable to their Red or to their Force carbon arms. Personally, I have a pair, and they look fine. If you are getting a Quarq spindle, I believe it has to go with SRAM or Quarq crankarms - the splined interface should be the same. If I'm correct, then, you could bolt Force or Red crank arms to the bike if you wanted to, for some reason.

There are other power meter options available. You could get a Stages, 4iiii, or similar left arm with a power meter. That is left-only power, and there's a decent chance that if you have other setups that measure total power (e.g. your road bike has a two sided meter, or you are comparing to a smart trainer), your numbers may not match exactly, but this may be something you can live with. You may even be able to ask either of the meters' firmware to offset your power (e.g. reduce it by a fixed percentage when reporting to the head unit). I would mention Garmin's Vector pedals, but I realize now that they're pricier than a full DFour91 DUB setup would be (including BB and installation). Other SPD-compatible pedals comparable in price to the Quarq may be released, however.

One last thing to consider is the crank's chainline. I believe you actually have a Diverge with GRX 810. (There is an RX800 rear derailleur with a clutch, but I believe that's the only component prefixed with RX.) This is an Ultegra-level set of components, but it has some different specifications. Specifically, its chainline is offset 2.5mm outwards to increase clearance. If you get a road DFour, you won't have the correct chainline. I think it would physically shift, and it may even shift well on at least some bikes, but it's not technically compatible. I would recommend a crankset with the correct chainline. I am not familiar with SRAM's offerings, but I believe that GRX has a 46.9mm chainline, and Ultegra uses a 43.5mm chainline. [SRAM's Rival Wide][1] crankset (wide for wider gearing, I think this option is also available for Force) has a specified 47.5mm chainline, which should work. I am not sure if there's a Quarq-branded equivalent, or if all Quarq cranksets use the road double chainline - Quarq is definitely favored by triathletes and I don't think it's as widely used in gravel, so SRAM may not have created a Quarq wide crankset.

In the unlikely event you are looking at a closeout DFour with a 30mm spindle, note this thread on [Slowtwitch.com][2]. Some DFours may have had an issue where if you dropped your chain to the inside, it would hit the meter in such a way as to damage one of the strain gauges. This would render the meter inoperable. Dropped chains do sometimes happen on gravel, even with a properly adjusted setup. I am actually one of the victims. SRAM did replace the cranks that were within the warranty period and corrected the issue in manufacturing. Notwithstanding that, I'd be more hesitant about BB30 DFours, because I don't know which production lots were affected by this, and you don't know that if it breaks, it will do so within the warranty period. I can, at least, report that I similarly dropped my chain on the replacement DFour DUB, and it continues to function normally (and FWIW, my front derailleur hangar had come loose, unbeknownst to me, which caused the drops).


  [1]: https://www.sram.com/en/sram/models/pm-riv-w-d1
  [2]: https://forum.slowtwitch.com/forum/Slowtwitch_Forums_C1/Triathlon_Forum_F1/_Quarq_Dzero_Failure_P6602771/