First of all, Spirit Swords don't stack. So your strategy you're imagining won't really work. Spirit Sword, when your CP threshold fills, wipes your mana, adds your MAX MANA (important, MAX, not current) + your level to your base damage temporarily. If you happen to fill another CP threshold, you just get the same effect again.
Personally I find Transmuter is one of those classes that is a bit unintuitive to play. Spirit Sword is extremely strong in the early game, but late game it can just get in the way. You can especially be tripped up if you're playing a late-game conversion class.
As above, the main point of the class is planning. There are basically three ways to use Spirit Sword:
- leveling
- make a non-popcorn enemy popcorn (bit of a niche use)
- extra raw damage against a boss
For leveling, it's all about planning. As above said, if you plan it out properly you can get two Spirit Sword strikes in on a single enemy without having to convert anything and without wasting mana (hint: start attacking when you are 6-9 points away from a CP threshold, with a full mana bar, depending on if you have GETINDARE and how much mana you have). Added to CYDSTEPP, or precasting ENDISWAL, and GETINDARE or just fireballs, this gives you a very high damage spike that should let you get level+3 or higher kills fairly easily. Just make sure not to get carried away with casting glyphs for this, because you want to keep some blackspace around for the boss fights, etc.
This is a bit specific, but if you're in the middle of a boss fight and need just a little bit more EXP for a level up, but are out of easy popcorn, you can use Spirit Sword to secure a level up.
Against a boss, there's less thinking involved. Just spend your entire mana bar and then go ahead and make your conversions for a nice +30 or so damage boost for each strike. The only thing to be careful about is to try and avoid reaching two thresholds at once (for example, converting a 100 CP glyph on an Elf when you already have over 40 CP) since you'll be wasting a Spirit Sword in that case. For this reason, More Glyphs is a good prep for this class, though attack boosters is also quite worthwhile.
Quick rundown on builds: Good items are bonus damage. Dragon Soul gets a special mention for earning you free CP. Human is obviously really nice and makes short work of the boss, but will have a harder time leveling. My favorite race for this class is actually Goblin, because the level ups override the mana wipe from Spirit Sword, making it much more convenient to play.
There is also the Infinimuter build, which isn't really related to actually playing the class but is pretty cool nonetheless. I direct you to this video by Choongmyoung:
https://www.youtube.com/watch?v=gJ1qehEXlLkTL;DR: Plan leveling kills carefully, and go for fragile enemies such as Goats, Gorgons, etc. For bosses, just spend your CP and be careful not to trigger the CP threshold more than once per conversion.
EDIT: Oh, and don't use LEMMISI too much. Unless you're in a map with a lot of unreachable blackspace, all you should be doing with LEMMISI is casting it a few times to find your glyphs and altars and then finding an opportune time to convert it and free up space in your inventory. Or if you're not hurting for inventory space, just go ahead and carry it til the boss fight.