It's "training" not "trainings." But, whether an -s or not, you don't have to do it all. Just what you are missing...
The FARs list the requirements. If you have met them, dual or solo as applicable, you do not have to meet them again. The FAA does not care what registration was on the airplane.
In addition to the specified requirements, you must be able to fly specified maneuvers to specified standards. In addition to being able to fly, you must pass the oral portion of each check ride before you get to show how well you can fly. The maneuvers you fly and what you say are rather different in FAA Land.
The comment above about doing the ME commercial and instructor in one check ride is wrong. You must be a ME commercial pilot to take the check ride. Ditto for SE. You can do SE then ME or vice versa, your choice. IF you meet all the FAR 61.129 requirements for ME as your initial commercial certificate, if you are more current in ME you may want to do that.
There is no need to do a flight review before you start training as long as the instructor is willing to eventually sign it off before you fly solo.
There is a requirement to for the instructor to certify three hours of flight instruction in the previous two months. If you are working on ME commercial and instructor, this could be the same three hours. Ditto for SE.
The 61.75 validation process will give you an instrument rating you can use with that private certificate. You cannot use it for an FAA commercial certificate.
So, find a good instructor to review the requirements and your logbook*. Identify what requirements have been met and what you need to do. There are great ways to economize. When you fly out/back to do the commercial maneuvers, fly approaches when you get back to the airport. Saves some money. Consider doing the SE commercial and CFI at the same time and take both check rides in the right seat.
Here's what I think you need:
Get a medical
Instrument written
Instrument rating
Commercial pilot written
Airplane flight instructor written
SE commercial in the right seat
Followed immediately by SE CFI (instrument rating required)
Your choice
Instrument instructor written and check ride - likely more marketable, or
ME Commercial, ME instructor (Maybe just ME commercial if you have plenty of ME time to be eventually marketable)
Get an instructor job.
Build time to 1,500 hours. Get a job that will pay for your ATP and type rating.
Good luck!
By the way - it's not a "conversion" despite the constant use of the term. Your EASA license is not converted into anything.
*This is actually all legit ground instruction for things you need to know for these certificates and ratings, so pay him/her for his/her time! It will likely take a couple hours to get it sorted and for both of you to understand what's required. Well worth it if you map out an efficient plan.