I have used Facilethings for over a year (just renewed for my second year last month). I am a refugee from the IQTell shutdown and tried several apps before deciding to stick with Facilethings (took me a bit to decide as I can suffer from paralysis by analysis).
If you are in the project view when you complete a next action you can hit the + (Add Action) in the lower right corner of the screen to add a next action to that project. You can also enter some of the parameters for the next action in the entry box while you are first entering the next action (for instance - if you enter @name in the box as you start the description of the next action it will assign it to that person). There is a list of parameters in the help section - I believe they are all the same as you would use when creating a task by sending an e-mail to the system.
The Android app has gotten better and is useful for basic functions when I am out and about. I also use an addon that lets me create tasks in the inbox using google assistant by saying "Collect" then whatever I want added to the text in the entry of the inbox. I use that feature at home with the google mini's that I have all over the house and when I am in the car through the bluetooth hands-free speaker.
I use the notes on each next action to keep track of everything related to that item (e-mails sent/received, phone calls, open help desk ticket numbers etc.) I date and time each entry in the notes and enter new notes at the top (a suggestion I made was a button that would add the current date/time at that spot). I keep reference and other files on google drive and link the key ones into the next actions for quick access. You can link them into the program but I like having them in google drive as it is easy to copy a whole project folder for someone else. Another suggestion I made was to allow tags/notes/files to be attached at the project level without a separate entry (you can create a reference action and attach notes to it - that's the way I do it currently).
I love the ability to use the people list for a quick way to find out what I am waiting for when I am talking to someone. I have their info entered into the system and use it many times as my contact manager to get their phone number to call them etc.. And right below their entry is a list of everything I am waiting for from them (that they have been tagged with). Makes it very easy to check on things and update the notes. If you create a people entry for them and tag them with an entry they can access the system through a web link even if they don't have FT.
The developer added subprojects in the last year (1 level deep). They work great as only the next action in each subproject list is shown on the working list. Once you do one the next one in that subproject is moved up.
I don't use the Kanban functionality but did play around with it to see how it works.
There are some things I would change in the system (unless you write it yourself there will always be things you would do different

) but I am very happy with the program. The developer is great to work with and when I made suggestions he gave me an explanation on each item (will consider for the future, works that way because... etc.). I would suggest you reach out to him with an concerns - he is easy to work with.
Before this gets too long (I can be VERY long winded) - I would be more than happy to help you with any questions or suggestions on how to setup and use the system (or anything else about FT for that matter) from my experiences with it. I can't say I always do it the correct way but I have it working and it is a key system for me (at this point I would be lost without it).
Scott