You need to figure out what sailing is like and whether you want to do it before investing too much into getting ready.
Why don’t you just go sail with SUP or SIU for 180 days and get qualified to test for 3rd Engineer. That will probably take one calendar year and cost nothing, plus you’ll make decent money.
That won’t get you STCW OICEW, but it will get you started toward it. I don’t happen to know how many days you need for that.
You can sail engineer on tugs, factory trawlers, ferries , on Great Lakes ships, or Inland without STCW.