Lang8, but it is (was?) locked to existing users.
iTalki, a service with a journal feature and tutoring options.
Hinative is more of a question service, but you can post in a category titled “Does this sound natural?” and get feedback. Also supports audio if you want to record yourself speaking.
The “Japanese Only” subsection of WK. Most (all) of the questions can be answered with as much or as little complexity as you want there. It’s an interesting place.
For practice doncr has an awesome practice tool, but it might take a bit of effort to get used to at first.
Alternatively there’s good old pen paper. I like to take some vocab from my wk lessons and just write out all the different forms spreadsheet style. Japanese has almost no irregular words/conjugations, so it’s pretty safe to practice this way. There are a few conjugation tools/dictionaries available online and even jisho can display them (click “show inflections”) so it’s not particularly hard to check your work after the fact.