A validation can only be extended once, so when yours expires you won't be able to fly EASA aircraft. The EASA exams themselves are valid for 36 months.
As long as you have 500 hours multi crew you can get an EASA ATPL.
If you have 500 hours on a multi pilot type - all you need to do is the skills test on that type.
If you don't have those 500 hours on type (it sounds like you've only got 350?) then you'll need to do a full type rating course plus skills test.
So yes, you can do any new type and get your ATPL, but once your valuation runs out you won't be able to fly your current type any more without passing an EASA skill test on it and either (a) getting 500 hours on type or (b) doing the full type course.