In the past, I used Slackware and Ubuntu. I dabbled with FreeBSD for a few weeks a few years ago, but I didn't want to compile KDE on a slow laptop. Now, I'm on XFCE and plan to move to a lightweight window manager like OpenBox, IceWM, or FluxBox.

Now, I want to migrate to Gentoo because it allows me to remove unwanted softwares including, but not limited to, dbus, systemd, and PulseAudio. I want to avoid softwares controlled by RedHat. RedHat softwares are too unstable for my graphics workflow. I'm not sure if dbus is as harmful as systemd, but I'd like to avoid it if possible. It was difficult to replace systemd with OpenRC on ArchLinux. I had to write a few OpenRC services. ArchLinux became somewhat brittle after replacing systemd with OpenRC. Removing dbus and PulseAudio in ArchLinux would be even more difficult, so I won't do it on ArchLinux.

I planned migration. I currently use chromium, but after I move to gentoo, I'll probably use firefox because it seems firefox can be compiled without dbus, gconf, systemd, PulseAudio, etc, ... on Gentoo. I will ditch LibreOffice by learning to write ConTeXt before moving to Gentoo. But, I won't be able to avoid compiling gimp and krita.

I wonder if My AMD Athlon II X4 640 is fast enough for compiling such heavy programs as firefox, gimp, krita, etc, ... I have 32GB RAM, so tmpfs could speed up compilation, but I estimated that each system upgrade will still be going to take 6~10 hours each week.

The question is how long it should take to migrate. I could intensely focus on migration for 2 weeks. But, I could also spread the migration over a year. I'd like to prepare the migration over many months in a virtual machine, but I find it difficult to do computer maintenance only for 1-2 hours every night. Once I start maintenance, it's difficult to stop thinking about it after 1-2 hours, and thoughts about maintenance percolate through my daily life. The path of least resistance is to just pour two weeks into this, but it's hard to justify pouring two weeks into one task because now I have study and social obligations that I don't want to delay.

How can I make the migration as smooth as possible?

what, 2 weeks? i was going to suggest 1 weekend... what exactly do you mean by "migration"?
i was thinking, save personal data, reinstall (i know this can be resource consuming for gentoo), add final touches...

It would take one or two days for a person who already troubleshot all potential problems of a Gentoo system or who doesn't have a lot of baggage from the past to carry with.

The migration involves a lot more than just installing gentoo.
The reason that I want to migrate to gentoo is to embrace new ways.
It is going to involve migrating from various other softwares. For example, I have to migrate system configurations written in SaltStack.
In ArchLinux, I can iterate over various softwares quickly, but in Gentoo, compilation slows down iterations.
I also plan to install Gentoo on ZFS, which is going to take more time because ZFS is not in the mainline kernel and I haven't installed a linux system on ZFS before.
Because I haven't used Gentoo before in my life, learning Gentoo ways, fiddling with USE flags, and recompiling various applications will take extra time.

Thus, two weeks are a conservative estimation. It would be too painful to run a marathon session for more than two weeks. I once poured weeks onto system maintenance before. It was very uncomfortable. I forgot to sleep enough, drink water, and take rests. I no longer want sustained pain.
Since my ArchLinux system is working, I'd rather spread the workload over months so that I can comfortably work out unforeseen problems, document the installation process, and migrate configurations in configuration management system in a virtual machine.

Also, from this opportunity, I want to learn to change a linux system without giving up other aspects of my life.