The timestamp difference is likely not 3 minutes, though that would be indicative of some sort of block withholding/selfish mining but I highly doubt so. It is far more likely for an inaccuracy between the timestamp. From my past experience, my node always received the blocks within ~1-5 seconds of each other which is congruent with the propagation time of the network. You can still use the timestamp as a variable if for some reason you need it though.

The main problem with SPV mining (or mine now, validate at the same time) was the major fork back in 2015 by the various larger pools which has largely resulted in a huge income loss by the larger pools. If you read back on the issue and the topic at that time, they actually didn't really care if they validated the block or not which resulted in a total mess that the community had to deal with. The point at that time was that they wanted to gain an edge over the others, not because of bandwidth constraints. It simply made more sense to just get a server that does it for you or to just join a pool. The profits from the block transaction fee far outweighs the cost.

I highly doubt that any farms would bother to not include transactions because those up/down speed would be okay for stratum, considering the fact that you can still cut down on unnecessary transmissions to the pool. It is still faster and more cost efficient to rely on an external server to compile and feed the data instead of pure SPV mining. With that in mind, most of the larger miners/pools used to use Bitcoin Relay Network or now known as FIBRE but I'm not sure if its still in use or replaced again. They actually do have an edge over the others by that alone though SPV mining was a prominent practice at that time. I think there was some proof that it actually didn't really made sense now with all the optimizations.

Also, at some point in time, the empty blocks might've been due to covert ASICBoost but it isn't happening now that overt ASICBoost is so prominent.

P.S. I don't think intentional sabotage is likely because that is quite expensive in the first place, only happened once accidentally.