Yes, a tech diver should always service his regs. Consider this as a part of training required for becoming a true tech diver.
Regarding availability of original parts and proper tools, in my opinion no other brands equates SP. I still find original service kits for my four old Mark V (that is a MK5+109) after 40+ years they were built!
And during the years I was allowed to update them adopting all the innovations occurred, such as a balanced poppet for second stage, blue silicon exhaust valve, the new hand-screwed hoses, the stainless steel retainer for the turret on the 1st stage, the new conical seats for the MK10, etc...
I think that with no other brand I could be still using them after 45 years.
A word about the "grey market" for service kits. I do not know exactly what you do mean, but here in Italy there are several physical shops who are authorized Scubapro dealers, so they buy the kit directly from the factory, and then they sell them on the Internet. When I did purchase them, I got also the regular "scontrino fiscale", which is the document proofing the payment for the Italian tax Agency. All legal, all official, nothing "grey" or "black"...
As these online offers are not limited to Italy, you can buy these service kits online almost anywhere in the world.
Here for example a couple of them, selling the MK25 service kit:
https://www.acquasub.it/en/catalogu...58/maintenance-kit-scubapro-mk25-af-sa-t.html
Scubapro Kit Revisione 1° Stadio MK25/AF/SA/T/MK20/MK25 EVO - TEK Evolution
The second is a bargain, just 18€.
If you happen to come to these shops, you can also walk in and buy the kit at the desk. I see nothing "grey" in this...