I honestly think they have the ARM chips for everything except maybe the Mac Pro ready to go as far as being designed and ready to mass produce already. I think the only reason they're staggering out the product launches is to not overwhelm the market with new products (because when do companies ever re-do their entire product line all at once?) and not to "rush" the transition by having ARM Macs release too close on the heels of their last Intel Mac equivalents (which as Apple said, there's still a few more coming) that were already in the pipeline before Apple decided to pull the trigger on the timing of this transition. I think it's more about timing and not pissing off customers more than they already might and not really that the higher powered chips are still in development or something like that.
I think the rumors of the Intel 13" MBP getting discontinued when the ARM variant ships later this year is actually spot on. It explains the 2 lower end tiers of the 13" retaining the 8th gen processors.
So in my theory, around November time-frame Apple will update their MBP lineup. The 2 lower tier intel 13" MBPs will get replaced with ARM variants while the higher tier 10th gen models stick around for another 6 months or so. The 16" will stay intel this cycle. Same thing I see happening with the iMac. The lower tier 21.5'" intel models get replaced with a new design 24" model while the 27" stays intel.
Doing it like this with ARM chips that will likely outperform the intel variants (even though the intel variants will be more costly) would give even more incentive for customers to switch to ARM while only developers with a NEED for x86 would buy the higher tiers.
Then early next year they update the Macbook air with arm processors (discontinuing the intel model entirely) and possibly revive the 12" model as the "budget" laptop priced lower then the entry model MBA. Same for the mac mini. The intel variant gets discontinued and replaced with the arm variant early next year.
That then only leaves the 16", the 27" imac/imac pro, and the mac pro which I would all expect to get higher performance chips.