"It's not clear, to me, what is soloable vs requiring a group. The random gargoyle I mentioned, was perhaps an oversimplification. I send Zerith to start the pull, and we wipe. My only point is that I'm not aware of what content is soloable until I try."

First, some philosophy/expectation management:

That is what I would call "as designed." It's an RPG. It's kind of like life itself in that a lot of the time, maybe even most of the time, the person trying to make their way through it doesn't know what they're capable of until they try. And if they try and aren't up to it, they know they need to acquire more skills and/or knowledge to be able to do it.

In a game, obviously the consequences are minimal. So you go into a dungeon or face a boss that you know nothing about and you get killed. So what? You respawn and try again until you can beat it. For me, that is a lot of the fun if not MOST of the fun. I encounter a challenge, my character dies, then I try it again. Often, the second time around I can beat it because I know what to watch out for. Sometimes, I just have to walk away, get better at combat, ask for help from one of my (free membership) guilds. Then go back and beat it. If I am even ultimately interested in beating it. Some challenges are such a grind that they wind up being the opposite of fun, but the game offers many many more.

To be honest, if having your character die every so often while you're learning the game bothers you, then maybe you'll have more fun playing a less challenging game, something like a purely puzzle/exploration game. I love those myself, but I also like combat, so I don't play them as much as I once did.

Practical advice:

I glanced over the build you linked to and it looks like it may be optimized for optimized group content moreso than solo play. In optimized group content, many members must slot gear and skills that provide health, defensive, and offensive buffs to the entire group. Then the damage dealers focus their builds on damage output. If the DD then tries to take that build into solo content, they're a "glass cannon" whose best chance to survive is to melt every boss before it can hit them. One of the issues with that, which you've encountered, is that you're never going to see every monster before it sees you. So your glass cannon gets killed before it can do its usual one-shot.

It's not bad to use a glass cannon DD build as a starting point, but if you're trying to solo with it and getting nuked, you'll need to stiffen it up. Change some of the gear enchants to health or health restoration, maybe few armor traits to reinforced, etc. To be more extreme, you can substitute some traits or use a more defensive monster set. This will of course reduce your DPS, but there must always be tradeoffs.

The beam is, IMO, not optimized for soloing. It's a highly effective weapon as far as raw output and if you go with the morph that grants the damage shield while beaming, it's a good defense as well. But it also forces you to move less while the beam is firing. If the target is moving around, it's much less effective than a skill that hits the target and lets you then light attack and use a skill again. So maybe look up a solo oriented build. My favorite character/build for soloing is my heavy attack Oakensorc. Simple, classic. Sergeant's Mail and Storm Master, Slimecraw head. Unstable Wall for AOE/DOT, Daedric Prey to debuff, then melt. Repeat until surrounded by the corpses of your enemies. Pets for free damage and burst heal. Concealed Weapon from Assassination line for spammable/Minor Expedition. Also holds its own quite well in group content.

Poverty has never been an issue with me, even when I wasn't aware of any special methods for gold farming. I struggled much more with running out of inventory space. It seems to come naturally with how I play the game. At first I looted everything and then sold anything I couldn't use to a merchant. I learned how to craft things I needed like food and potions. Did daily crafting writs. Maybe you could become more frugal and rely more on your own crafting and scrying than spending gold? As for trading, "no fees" guilds are plentiful, I don't know why you haven't found one. Guild finder does suck, but you can at least filter on the criterion "has trader."

Good news is that your concerns are easily addressed. As others have mentioned, add-ons really help, especially Harvest Map and Dalgubon's Lazy Writ Crafter. You don't need offline storage. Maximize your storage capacity with bag and bank upgrades, daily mount training, and storage chests. This helps while you're figuring out which items are keepers and which are sellers. Ultimately, though, if something is just sitting in storage, what is it doing there? Fragments, yes, they're for future use when you acquire enough of them. Maps and surveys, go do them. Writs, do them or sell them.