In Romans 11:36, 'of him and through him and to him' refers back to 'Lord' in verse 34. But is there more than one Lord ?
In the 110th Psalm, David says :
The LORD said unto my Lord ... [Psalm 110:1 KJV]
Sit thou, said the LORD, at my right hand, which is in heaven.
Then a voice speaks to David ...
The Lord at thy right hand... [verse 5]
That Lord is at David's right hand, on earth, saith the voice of a Spirit.
These are divine mysteries, known only to those who, like David, experience them. And they are written for our learning. That we may perceive aright what it is that we experience, in our own day.
The word 'Lord' in Romans 11:34 conveys a title for Deity. And in the New Testament scriptures that title is most commonly ascribed to Jesus Christ throughout the books of the Greek scriptures from Matthew to Revelation.
Specific to this question, Paul, in the epistle to the Romans, applies the title 'Lord' to Jesus Christ about eighteen times, almost all of those being the direct title 'Lord Jesus Christ'.
Elsewhere in this epistle, written to all that were in Rome, beloved of God, Paul uses the title 'Lord of Sabaoth' and refers to Lordship in respect of the whole earth (Romans 10:12 and Romans 10:13).
But there are occasions, and this is one of them, when Lordship is not confined to a particular person (the Father or the Son, or the Spirit - who, on occasion, is also referred to as 'Lord'). One might see this in the previous references I have made, or one might suggest that the Father, only, is in view. Personally, I would say the former, but I would not expect everyone to agree with me on that.
'Lord' in verse 34 refers back to 'God' in verse 33.
The particular subject of verse 33 is wisdom and knowledge : its depth of richness within Deity. That is to say within Deity as such. Within Divine nature. Not exclusive to any person within deity - it is so of divine nature as such.
Thus I do not see that the word 'Lord' in verse 34 refers to the Father. I see it to refer to divine nature as such : to the Lordliness that is appropriate to divine nature due to the depth of the riches of wisdom and knowledge within that divine nature, a depth of riches that is common to divine nature, as such, and not peculiar to any one person, in particular.
Any one person possessing that divine nature, will have that rich depth within their being : a rich depth of wisdom and knowledge within the divine nature which they possess as the means by which their being is expressed, the consequence of possessing such capacities being : Lordship over all that which does not possess that characteristic of deeply rich knowledge and wisdom, and many other characteristics also.
Thus, no, I do not see that the Father is being singled out, in this particular case.
'For' connects an argument with a previous statement and progresses to the conclusion of the argument. This is so because of ... something else.
'For' follows on immediately from the previous wording ... and then rests on the next statement as a conclusion.
'For who hath known' refers to 'the depth' which is 'unsearchable'. So, nobody knew. Because of the depth.
And the depth is within deity, who is Lord.
But some might say that this is just an opinion. That the difference between seeing 'God' (that is to say Deity) or seeing 'The Father' as being in focus, is just an opinion. I would say not so.
But if the majority were to say it is so, then perhaps the question is asking for an opinion, in which case the majority might think the question is opinion based.
But I would simply enquire that if Paul meant to say 'Father' then why did he not say so and why did he say 'God' . . . and further refer to 'Lord' ?
To me, the reason is because Paul is conscious, not only of Deity who is Lord of the whole earth, but is also conscious of Deity as such, the nature of Deity as distinct from Person.
Paul can distinguish when to refer to the Deity of the Father (his nature) and when to refer to the Person of the Father, whose nature it is.
Which very fact reveals a truth about Deity. Were there but one Person in the Godhead then were there no reason to distinguish between nature and Person, to distinguish between 'God' and 'Father' and 'Lord'.
He would be but one Entity and these terms would not apply.
God would be an unknown spirit, with no way of communicating with human beings. And with no way of saving them in their humanity.
Thank God, it is not so.