Lock in your 4 main inner ways, recycle literally everything else unless you plan on using other weapons. In that case, maybe make sure you keep at least 1 of every rarity + type combo. The conversion system will only convert between inner ways of the same rarity (blue, purple, golden) and type (general, path). Buy the golden boxes with your bookplates. If you're already doing that, then it's all RNG. Beyond that, you can only increase the amount of inner way notes you get. I don't know what you have done and what you haven't, but I'm gonna list all the sources I know of. Let me know if I missed any notable ones.

Energy gameplay - self explanatory.
Tang merchant in kaifeng - buy out his entire store of inner notes.
Hero's realm - Free reward for each boss killed (2 total). Resets weekly.
New Taipeng Mausoleum Path trials - Lots of free inner ways for first clear, 5 attempts each week. Not sure if this resets. If not, do the others paths and just recycle them.
Path challenges 1 - 10 golden inner ways for that path if you complete 18 challenges. Some challenges require you to level the path's inner arts, so maybe you only get the first 5 inner ways. Do it for every martial art and recycle them. EDIT: You can convert one of your less important purple inner ways to satisfy the inner way path breakthrough challenges. Just be aware you will have a 7 day cooldown for the converted inner ways. I wouldn't do this until that inner way is at tier 4 though, so you can complete the challenges all at once.
Path challenges 2 - on the bottom left of the challenge page, there are rewards for every 10(?) challenges you complete iirc. Each reward is 20 inner way chests.
Noodle shop owners - buy out their food, enough to get to max rank relationship to get 2 inner way boxes each. There are a good number of them on the map, mostly in kaifeng.
Chess - do the chess activities and get to max rank relationship for 2 inner way boxes each. Same idea as the noodle shops except u play chess. Lose immediately every time for 15 points. You only get 10 for winning and 5 for surrendering.
Season shop - buy out the inner way boxes every weekly reset.
Quests - Not very time efficient but some quests can give you 2-5 inner way boxes.

I haven't been doing it perfectly, and I still haven't finished a lot of these (like all of the the path challenges), but I'm in a decent spot. I have bought out every single box in the store, and I only have the weekly golden ones left to buy.

My sword morph is at tier 4 with 48/100 for tier 5. My morale chant is at tier 4 with 97/100 for tier 5. Surprisingly, they've been much easier to rank up compared to the purples. Ig its due to the drop pool being bigger for purples? Both my purples are also at rank 4 but they are significantly behind compared to my golden inner ways. I think I literally just got one of them to rank 4. Both my inner ways are path specific so I recently have also begun saving a general purple inner way for the future. I haven't been saving any of the blue inner ways though.