HSR is too strong

Okay. I really don't know how to begin so I'll just get into it as best I can.

Healing was never, will never be on par with HSR. I'm inclined to agree with Zelda on that assumption, healers cannot heal mid combat whereas HSR allows for that. But at the same time let's be careful not to compare apples to oranges here, healing at least for the shinigami as far as canon goes was most effective out of combat. If you look at the cases where injured shinigami are healed even in the anime their comrades would drag them out of the battle scene and heal them away from the carnage, it should even be mentioned here that Unohana herself, aside from the fight she had with Kenpachi, stayed away from the front lines in order to heal Hiyori and co. during their fight with Aizen. Like Zelda said they shouldn't be the same, barring the time it takes to regenerate one limb being 3 posts for both.

Practically speaking I really don't like Zelda's example, I say practically because whether you're a shini or hollow, losing an arm mid fight is a serious disadvantage and in all honesty, like the battle guide says, is a death sentence unless the afflicted party removes themselves from combat in order to deal with it. Also unless your opponent is a sadist I doubt they'd wait for you to heal in order to cut off the same arm again, unless they were that much stronger than you in which case it wouldn't really matter either way.

And whilst you might say its just an example we also have to consider that, whether you're an vasto/car or a shini, limb loss is just as grave in combat, again unless your opponent is a sadist they're not gonna just stop their assault and let you heal yourself, on the flip side unless your own character feels no pain/ has no fear of death they're not just gonna continue like losing an arm was nothing. The main point of this being that even with HSR, healing for shini and hollow is most effective when not in combat.

In regards to canon, if we want to be real nitpicky here (Bleach 351 pages 7-8 for those of you who want to read) he didn't say he was the only one who choose it, or that it even reduced your power, but rather it could be exchanged for even more power, and that he was the only one capable of regenerating anything but his brain and vital organs. Even Wonderweiss himself was able to regenerate at such speeds against his albeit very short fight with Yama (Bleach 394), whilst other arrancar such as Nnoitra and Redder were shown to regenerate in their resureccion forms. That being said even with the suggested 'power drop', Ulq was still able to kill Ichigo with relative ease twice so it's still sketchy as to how significant a factor that plays in the overall power of a vastocar.

If you're still with me up to this point, the tl;dr here is that hollow do have HSR and that they trade it for even more power when they go arrancar.

That aside we can now come to the solutions.

I'm against making HSR into a mastery, mastery implies skill and training which to me isn't really applicable to regeneration since everything regenerates as part of the biological process from humans, to shini and even bount though they accelerate their with the use of reishi which requires mastery of reishi hence why it's a mastery. Shinigami accerelate this process through use of healing kido and/ or their release which also requires skill.

This process of regeneration is naturally faster for hollow than the other races hence the name HSR and is why it's considered a racial quirk (like melanin for nigg--) rather than an actual mastery. In this way it will always be 'free' to hollow the way speed is 'free' to a cheetah (poor example but you get the point).

That being said it does need to be reworded, especially where arrancification is concerned, I've already said how everything regenerates as part of the natural process but Zelda presented a dilemma of sorts of how one's natural process can exceed that of a trained master or specialist which shouldn't be so. I've also said that regardless of whether healing by a specialist or with HSR both are most effective outside of combat.

In that regards HSR be 'dumbed down', we've seen arrancar like Nnoitra and Wonderweiss regenerate whilst in resureccion, so maybe it could be amplified whilst in Ress but there needs to be a distinction between hollow that keep ress post arrancification and those that choose a power boost. Using Ulq as the example we've seen the extent of his regeneration whilst still keeping a formidable arsenal (tossing lanza del rampagos like javelins) maybe those that sacrifice HSR get a much more significant power boost in terms of site mechanics and only get to use a weaker form of HSR during ress whereas those that keep it get to power it up to Ulq's level during ress.

In regards to healers, I say no to healing in combat, simply because as I've said healing for both HSR Vastocar and Shini is most effective outside of it, but to compensate allow healers to heal from the sidelines I know there's already something like in the spec perks but expanding on that to make it more effective, a healing aura wouldn't be a bad idea either. The same way an arrancars hsr grows faster in release a healers healing prowess would also increase whilst in release so we get Unohana levels of healing where you can bring someone from near death back to perfect health almost instantly [like her fight with kenny].

the tl;dr of this post can simply be you can't compare healing to regeneration, it's like apples to oranges, one is naturally enhanced whereas the other requires skill and training to enhance, rather instead introduce more perks that suit the healers role [outside of combat] rather than try to equate it to the hollows HSR which will need to be dumbed down to account for the current disposition.