I might have a different train of thought on the character due to having taken Zenurik as my Focus and preferring to ignore melee in most cases, but if you're asking for opinions here are mine:

--- Mods

Straight up, when you have the option to, get rid of Enemy Sense for her Regenerative Molt Mod. I never have a problem knowing where enemies are on Sayrn, since they always bleed numbers onto my screen.
I focus on Armor and Health thanks to that mod—the major benefit of shields is that they regenerate easily if you take damage (allowing you to abuse enemy's tracking/accuracy and then regen back up), and the Regenerative Molt mod makes it easy to replenish the superior health stat. Sayrn has a moderately high Armor Rating. Without Steel Fiber she has 175 armor which is ~37% damage reduction while Steel Fiber brings her to 367.5 armor which is ~55% damage reduction. At that level of damage reduction from armor and the sizable buffer you get from vitality, healing 50 health per second for 10 seconds with Regenerative Molt is pretty fantastic.

With three slots already taken up by Defensive mods, I next want to add Stretch (which you already have equipped) to increase my chances of spreading Spores. I'm not convinced that Over Extended is necessary, but I also don't have the mod, so I can't play around with it myself.
Sayrn is in a weird position where Range = Damage. Getting all enemies maxed on toxic spores is her damage (Making sure that they're Toxic is especially important against Corpus). Even without the toxic proc spreading toxic spores, spores (eventually) halve the health of all enemies affected with the viral status effect.

After that I think you can mostly get away with doing whatever you want. Primed Continuity and Primed Flow are both solid choices.

Theoretically: I like Steamline instead of Fleeting Expertise —since casting Spore on Molt halves the cost of Spore (I have observed the in-game-cost to be 9, the math puts it at is 8.75, but you need a minimum of 18 to cast it).
If you do use Fleeting Expertise, Rank 4 (50% efficiency, -50% duration) is probably my preference. The energy cost on Spore should then be at its lowest point of 6.25 (but you still need a minimum of 13 to cast it).

--- Ability use and playstyle:

I love Molt.
Slash Status? Molt. Toxic Status? Molt. Damaged? Molt. Someone looked at you funny? Molt.
Health Regeneration (with mod) and Status Cleanse and things like to shoot at the molt object more than at me and it makes spore cost less if it's the target of the cast. What a skill! I feel like this thing quintuples my survivability.

After using Molt to spread initial spores, I then use my Radiation + Toxin + Toxin status chance Hikou Prime (which will probably be changed to Veykor Marelok or Lex Prime or other weapon when I get either) to apply Toxic status to any enemy —who then often gets shot by other enemies (thanks to Radiation) and pops Spores which affects other enemies, who I then shoot too.

It's pretty rare I use either of my other abilities, but I use Molt and Spores quite often. I should probably use Miasma more often than I do. As for Toxic Lash, the energy cost —on top of the skills I already use— and my lack of confidence in melee keep me away from it.

The first 2m30s are still a bit rough —manageable with restores— but because I have Zenurik I normally have enough energy to keep this strategy working. You don't have to constantly cast spores if you're spreading them well enough. The duration of Regenerative Molt = 40 Zenurik Energy. With Streamline you're coming out ahead 5 energy. If you're not using energy constantly, Flow can let you have a higher max pool to draw from later.
Alternatively: I hear Trinity is a cool warframe.