Uneven VTF on different part of same LP

From the information supplied, some possible causes include:
1. with most tonearms, including yours, a non-level tonearm bearing vs. a level LP surface would result in a gradual change in VTF as the tonearm arcs across the record;
2. differing position of the tonearm wires as the tonearm arcs across the record could cause VTF changes;
3. dirty tonearm bearings; and/or
4. worn tonearm bearings.

Only #4 would require replacing or seriously rebuilding the tonearm. If you're happy with it otherwise, a thorough diagnostic to identify the actual cause(s) of the changing VTF would be in order prior to expending significant monies.

Start by checking the arm's freedom of movement. Reduce VTF to zero and disengage anti-skate so that the arm/cart floats free and level. Brush the arm gently inward and outward with a feather or a breath of air. It should move freely and equally in both directions. Any resistance could be evidence of 2, 3 or 4 (or a combination thereof).

P.S. A unipivot is still subject to 3, 4 and (especially) 2. There's no reason to believe a unipivot would automatically eliminate your problem.

(1) How accurate is your scale?
(2) How accurate is your level?
(3) We have too much faith in manufacturers and subsequently believe that their products are manufactured perfectly. Not so. Unless you're willing to spend big bucks on products manufactured to "precise standards, these types of issues will always crop up.
(4) If your bearings are worn/not perfectly round, that is another issue.
(5) How accurate is your set-up, considering all of the above.

Vinyl reproduction requires a lot of thought, proper set-up and products manufactured precisely in order for the end user to obtain satisfactory performance.

Further I put bubble level on the tonearm's bearing housing, it showed unlevel (the turntable itself is perfecty even) which might cause the above issue.

Well, when an Armboard is not level, you will ruin your cartridge pretty fast.There are so many different forces for it, it can't survive. Your cantilever will be bent sooner or later. Well, of course it is possible that you have also different VTF based on that, but the other problems are much more serious. Did you also hear a shifting in soundstage in the last 3 tracks?
Is it a DIY board?

If you have a digital scale it might be a good idea to change the batteries. Best of Luck and post up and let us know if you find the problem.