Thermal's Phytogenic Insolator cannot be used for the flax you can get via bonemealing grass or purchasing from the market. It can only be used on the flax seeds you receive via villager trading. This restricts automation of String/Wool until you get a T3 permit->villager trades, and then spend a significant amount of $$$ to purchase villagers, level up farmers, and pray for the flax seed trade for Thermal Cultivation's flax seeds as opposed to Supplementaries' Flax seeds. The alternative of using Industrial Foregoing's Plant Sower/Gathering is technically viable as well, but is also extremely slow without spending $ via admin-shop to for bones->bonemeal to use the Plant Fertilizer from the same pack for automating bonemeal usage on your flax plants.
Alternatively, provide an easier method of getting Thermal Cultivation's Flax, or a faster/easier method of farming string/wool.

While I could complain/feedback about how difficult it is to make AE2's ME controller, I actually think it's a good thing, and think RS's crafting tree should be nerfed to be similar in scope. The main issue with AE2's crafting tree is that a decent portion of it requires multiple steps from other mods from the ground-up that isn't already used previously and can't be repurposed, to craft things that is exceedingly involved compared to absolutely anything else in the modpack currently, and I'd like a couple things like the difficulty of creating string/wool to be eased/smoothed out a bit.

This specific feedback is about how difficult it is to grind out Flax, with either setting up an automated farm via IF which has a direct $ cost to speed it up compared to using a phytogenic insolator which only has an RF cost, as wool is required to upgrade your cables to covered cables, to make dense cables, to make your quantum rings to make your first me controller. Nothing else in the current modpack state requires large/automated amounts of wool, and it feels somewhat bad to be hard-gated behind manually grabbing 800+ flax, or setting up a crafting tree specifically for ae2's cables when it's not required for anything else in the pack I've been able to determine(except for the main crafting/questing path, which is entirely expected and liked)

Close; You can get Thermal Cultivation Flax from bonemealing, but it's extremely rare compared to supplementaries flax seeds. 100 bonemeal provided 5 thermal cultivation flax seeds total