You can get the spectral cloak from normal BrP, since that's how I got mine

However I don't use them at this very moment, since I don't want to and prefer other stuff. And that's exactly how PVP works in ESO. Obviously there will be best in slot gear, but there is always the question wether the best in slot for someone else is also your best in slot. PVE is way more set in stone than PVP. I feel like the pvp builds when you google them are not the only viable ones. It's not because your build isn't anywhere online, written down by a youtuber, that it can't still kick ass.

The advice often given in these kind of situations is to just try stuff out. That is an expensive advice and requires a lot of time to collect gear etc. Try to narrow it down and think/talk about it on these forums or with people in Discord/in game. For example when you run a setup, make a small change. What do you feel? Stam bar always drained? -> get some sustain, hitting like a wet noodle -> try to up some damage etc. This kind of thinking will get you to a setup that suits you and give you understanding of your class and playstyle. And it is that in the end that determines how good you are.

Alcasts build will still be playable without spectral cloak, but when min-maxing it's these kind of changes that make the difference in the end. Will you work for full gold enchants or settle with purple ones, go for the perfected spectral cloak or settle with the regular etc. To me, the difference can sometimes be in 1 line of gear or 1 change of enchant type. On the other hand: if you have to work so hard to get a certain gear piece, since it's supposed to be overpowered, you're sometimes absolutely dissapointed because you aren't suddenly performing like your favorite youtuber who's a killing machine. The reason that guy is a killing machine, is partly his gear but even more his knowledge about his class, character and game mechanics.

I'll give you a personal example: I have no mythics, so I have to run with alternatives. I also don't buy all DCL nor do I have ESO+ ... feel me coming? I'm missing out on a lot of gear that is considered BiS. For the moment I'm running 2 bloodspawn, 5 ravager, 5 daedric trickery and a VMA resto backbar. I'm running dual wield/resto as a stamplar, trying it out with the new update. I feel perfectly fine and often perform very well in both my scoring and my damage. With the resto even my heals are going insane now. I'm nowhere near the best player, I'm not even as good as I used to be but I'm enjoying myself and I kill people without getting nuked. It's the first time in years I play stamplar with no 2H, I feel like I'm just going half magplar. So you could say for example 2H has been BiS for a long time on stamplar, because of the skill rally. Which 2H you used, was a lot less defined. Yet people still used to name certain sets as absolute BiS. I've seen a lot of builds online with all sorts of 2H I couldn't acces, yet you work around it and go kick ass!

To conclude I'd say that the gear you have can help a lot, a lot lot. But it will be your player skill and understanding of the combat that defines how good you actually are. Certain things like line of sight, movement, knowing your attack/defence combos and those of others etc. will help you so much more. That's how you work around the imperfections of your gear. Make the imperfect setup for someone else your perfect setup by learning to play with it. Stamplar is a very hard class to learn, magplar is much more forgiving on that matter (I feel that with the resto staff now, near death? Nah mate, let me just boom my health up with 2-3 clicks).

Sorry for the wall of text, but I do hope it helps you enjoy playing without the stress about getting certain gear!

I wish you the best of luck!

In addition to that, I do watch a lot of youtube videos on stamplar builds. Just to get an idea of what people run, listening to why they do helps a lot.

Some people I definetly admire for their skill on stamplar are Asayas, Kuro and The Real Godzilla. Just checking out the vids, reading some comments helps a lot. It's also definetly worth it to just check out any video you find, since stamplar builds online tend to be scarce sometimes.
When you don't have a particular set or don't like it, these can give you ideas since other people might have the same issue.

For example I hate the Clever alchemist potion cooldown stuff, makes me go too crazy on the timer and mess up. Instead I use ravager. They have a similar way of working and they're both proc sets that give massive wpn/spell damage. The requirement for the proc from ravager is just so much more my style. Hence I play better... Of course in some cases you'll just have to deal with it and learn the new style or way of playing, but again that boundry is personal.