Zariman bounties, railjack is painful but works (excess parts can be broken into Endo). Dissolve dupe mods.

If you don't care about it being solo you can do the arena on Sedna( the one that starts with a v), this is better with loot frames and a mod drop booster since it works on Endo. Only one person needs the booster for it to apply to the whole squad, so sometimes a group will take you as long as you have a built Khora or Nekros. This farm is way better on steel path.

Arbitrations are also good since the rewards table features Endo and ayatan statues. Can also trade the vitus essence for galvanized mods and sell them for a small plat grind.

I like railjack, it's maybe not the best for pure endo but it also gets you credits (which you may also need for upgrading your mods) and a bunch of other things at the same time.