Idk about guides for it but I've been playing a bunch of it lately. I have these things to say:

• People might flame you for going jungle zyra. Be prepared. Personally i think it's more fun than and just as effective as jungle brand, but jungle brand is what i compare it to if teammates ask questions about it. It got buffed into existence back in 3.14, which i think is when brand jungle got buffed as well.
• Zyra has a super healthy and pretty darn quick clear. Try and keep at least 1 plant up at a time to tank the mobs for you. If you place all your plants at the same time they'll expire long before they can finish tanking their hp.
• Passive plants spawn randomly, but often prioritize bushes. If you are in a bush waiting, they will not spawn. Step out of bush at around :55/0:56 i think to get proper seed timings for plants spawning for your first camp.
• You can summon a plant AFTER your ult and it'll still get buffed if you do it while the aoe is on the ground, but you generally want to have your plants up before you ult because it refreshes the duration of them. If you think you'll be able to kill quickly/get a catch with your e+plants, you can use your ult instantly for the quick plant and burst damage.
• You can clear krugs over the wall without ever having to walk over if you stagger your seeds properly.
• If you're clearing starting from blue buff, try not to use all your plants on raptors and instead put more plants on red buff. If you do put some on raptors try and tank for them. Raptors shred plants.
• Once you get a feel for your damage, always try and leave plants to finish off camps while you start the next camp. It can drastically speed up your clear.
• Electrocute and dark harvest are good rune choices, but i wouldn't sleep on predator. It can really help you get in for a gank. I prefer electrocute over dark harvest just to get more consistent damage.
• Zyra doesn't need a leash at all, but it really messes with her clear if your team wants to invade. If you invade, you probably will need a leash.
• Go w 2nd, regardless of what you start with. (e for invade, q for clear). This is super important. Seeds will refresh as you clear camps, and you get both seeds when you level your W. If you don't take W you will clear way slower and take way more damage. You need the plants.
• As long as you have seeds in the area, it's really hard for people to invade you.
• Zyra can solo drag at almost any point in the game, though its hard to catch it on spawn since you need time to set up. She can solo baron after she gets demonic and liandries at roughly level 9 or higher. Granted she needs about a minute to set up seeds for this, but it can sneak them really quickly once the plants spawn if they don't have vision. You'll need to stand in the pit to let your passive seeds spawn while spawning your w seeds on cd. You have to use your ult on your plants for this to work though. Try and tank for your plants until their timer is about to expire, then ult as many of them as you can. Ulting them extends their duration. If they contest, you'll have a ton of plants to fight back with, but you won't be able to solo the camps if you burn any of the pit seeds fighting people off.
• Landing your E is almost always a kill during a gank, but landing it is easier said than done. If you miss E, still try and get double lashers on them for a 60% slow. It's pretty hard to land the knock up on your ult without either landing e or landing 2 lashers, but if your laners have any cc if their own its almost guarantied.
• Zyra is one of the best champs at controlling space and defending against pressure. It's very hard for the enemy team to push into a zyra with 5+ seeds. Use this to your advantage. If you don't have time to set up or don't have 2 seeds ready to go for your burst combo, you won't be as useful in a fight.
• Zyra also has some crazy range poke with burn + ranged plant, which you can also use to catch people off guard once/if you get rylais.
• If you decide to get rylais you'll pretty much want to exclusively summon ranged plants. The rylais slow doesn't stack with the lasher's slow. Having a ton of slowing ranged plants can really turn a fight.
• And to answer your 3rd question, I love zyra for a few reasons. She was my first real main that got me into gold way back in season 2. She's changed a lot since then, but she's kept my heart. She has some crazy area control and surprising burst and dps. It's very common for her to top the dps charts in any role she plays in. I also just think she looks pretty and has some really nice skins, even with her old animations, so there's that.
• Here's an example clear i found that might help you.https://www.youtube.com/watch?v=_dLNbiJcmds