Conversion/justification should not be divorced from discipleship/sanctification.
Rom. 4:5-6 should be harmonized with Rom. 1:17, 5:5, 8:9 and Eph. 2:8-10, etc.
It's not about conversion/justification being divorced from discipleship/sanctification. The one who is sanctified is also justified. (1 Corinthians 6:11 - And such were some of you. But you were washed, but you were
sanctified, but you were
justified in the name of the Lord Jesus and by the Spirit of our God. Those who have been justified by faith and are converted have also been sanctified
set apart/made holy in standing before God positionally in Christ and are disciples of Christ. Romans 4:5-6 is in harmony with Romans 1:17, 5:5, 8:9 and Ephesians 2:8-10 when properly interpreted and justification does not get conflated with ongoing sanctification.
We also see progressive or ongoing sanctification in which the reality of that holiness becomes more and more evident in our actions, words, thoughts, attitudes, and motives.
1 Thessalonians 4:3-4 - For this is the will of God, your
sanctification: that you
should abstain from sexual immorality; that each of you should know how to possess his own vessel in
sanctification and honor.
So, becoming
washed, sanctified and justified in Christ is a one-time event, yet abstaining from sexual immorality is not a one-time event. The believer possesses a positional, judicial standing of righteousness in Christ and, second, an ongoing process of growth in practical, progressive holiness which becomes increasingly evident in our actions, words, thoughts, attitudes, and motives.
A passage teaching this truth is Col. 2:6-7: “Just as you received Christ Jesus as Lord [kerygma], continue to live in him, rooted and built up in him, strengthened in the faith as you were taught [didache].”
Like a tree establishes deep roots in the ground and draws nourishment, a believer is to be rooted and built up in Christ, which strengthens our faith.
There is no qualitative difference between faith that accepts God’s saving grace at conversion and faith that accepts God’s working grace while walking/living (Eph. 2:8-10, 1Cor. 15:10, 2Cor. 5:7, Rom. 1:17), but only a quantitative difference as each additional moment passes–and of course faith remains non-meritorious during the saint’s entire lifetime.
There is a difference between receiving Jesus Christ by grace through faith at conversion then afterwards living our lives by or out of faith after conversion. It sounds to me like you are conflating justification with ongoing sanctification.
Grace is God's unmerited favor in which He shows us mercy, kindness, and patience instead of the judgment that we deserve for sinning against Him. God's grace cannot be earned by our actions/performance/works. Grace is based on the character of God and not on our sincerity, performance, or ability to keep the law. Otherwise, grace would not be grace.
Romans 11:6, "But if it is by grace, it is no longer on the basis of works, otherwise grace is no longer grace."
Galatians 2:21, "I do not nullify the grace of God; for if righteousness comes through the Law, then Christ died needlessly."
2 Timothy 1:9, "who has saved us, and called us with a holy calling, not according to our works, but according to His own purpose and grace which was granted us in Christ Jesus from all eternity."
God's grace (enabling power) also operates sovereignly in the lives of believers.
1 Corinthians 15:10 - But by the grace of God I am what I am, and His grace toward me did not prove vain; but I labored even more than all of them, yet not I, but the grace of God with me.
2 Corinthians 1:12 - For our proud confidence is this: the testimony of our conscience, that in holiness and godly sincerity, not in fleshly wisdom but in the grace of God, we have conducted ourselves in the world, and especially toward you.
2 Corinthians 12:9 - And He has said to me, "My grace is sufficient for you, for power is perfected in weakness." Most gladly, therefore, I will rather boast about my weaknesses, so that the power of Christ may dwell in me.