remain-lack
do all these sentences make sense?

What only remains to be done is..

The only thing we lack to do is...

what is only left to be done is..
The only thing remaining to be done is..

Spanish : lo único que nos queda por hacer, sort of "we only need to do one thing more"

Question

remain-lack
do all these sentences make sense?

What  only remains to be done is..

The only thing  we lack to do is...

what is only  left to be done is..
The only thing remaining to be done is..

Spanish : lo  único que nos queda por hacer, sort of "we only need to do one thing more"

Answer

You can't use 'lack' in this way. It just doesn't work.

'The only thing remaining to be done is..'   is OK.

or, more simply

'The only thing left to do is ....'

Answer

All are ok but the one with "lack"  it can't be followed by an infinitive. You need a noun of some sort, an object or a gerund.

"What is only left" is awkward with the double use of "is".

"The only thing remaining to be done is" is the best choice.

Or reposition the only in your first sentence: "What remains to be done is only to"