So, I will likely skip the Tamron SP 90/2.8 VC Macro (F017) lens for Fujifilm GFX - and go instead for the Pentax SMC FA 645 120mm ƒ4 Macro lens.

What is known about this lens:

• Image circle is larger than the Fujifilm GFX sensor (because it is 645 mediumformat lens)
• A highly regarded lens by many, known to render sharp images. Top at ƒ11
• It can focus down to 1:1 macro without tubes
• I can use it on my Pentax 645N (film) camera, too
• The lens does extend physically when focusing, though
• Also performs well at infinity
• Performs well in the photo studio like a 95 mm portrait lens

A safer bet
for scanning mediumformat negatives

I believe this is a much wiser choice compared to buy a second hand Tamron SP 90/2.8 VC Macro (again) - which has more limited use (e.g. basically only ideal scanning 6x7 negatives when using a GFX camera). I was otherwise not really impressed when using it for other things than Macro (on the GFX). The performance for normal photography at infinity was so-so. Nothing to write home about.

The Pentax 645 Macro lens however, is one with very good reputation.

Price

The Pentax 645 120mm ƒ4 Macro - costs in total incl. tax, shipping etc around 4800 SEK / 425 €
The Tamron Macro from eBay Germany is 5600 SEK / 500 € incl. shipping (pretty expensive, really).
The TTArtisan 100/2.8 2x Macro lens I just sent back, was 4550 SEK / 405 €.

Albeit - I need to buy a Pentax 645 to GFX adapter - which adds another 66 €.

Value vs usefulness

In my opinion, I think the Tamron Macro from year 2016 was and is totally overpriced at the second hand market (eBay) for unknown reasons. The AF seem to be relativelt prone to break, since there are several ones offered with "burned out AF motor" - but fcontinues unction as manual Macro lens. The same happened with the one i bought here in Stockholm, where neither AF nor IS worked.

So, it went back.

Also; i didn't like the performance of the Tamron Macro at infinity, where it vignettes on the GFX. Overall I didn't really look that good at infinity. Not as bad as the TTArtisan 100/2.8 did, though. Optically at infinity it was nothing to write home about, really. There was no love, when i saw the results at infinity, thereby limiting its usefulness.

The only advantage the Tamron Macro had on the GFX, was that it didn't vignette at 1:1 Macro - and showed to be sharp all the way into the corners when stopped down. That i thought was pretty remarkable.

That was about it.

The Pentax 120 Macro (both A and FA versions) on the other hand - are very attractive priced - known for their robustness and high optical quality. So, some people who use the different Pentax macro versions (A, FA vs D-FA), say that all three are very sharp, perhaps the the D-FA version being a notch sharper, but with rather little difference overall.

Why i hesitated in the past

I knew about this Pentax 645 type of 120mm Macro lens for a long time. And knew about people regarded it for both macro photography as well scanning larger negatives when using a Fujifilm GFX camera.

I hesitated simply because I wasn't interested into buying another adapter in order to adapt also Pentax 645 lenses (of which i have 2). I thought there was a better / more handy option for scanning larger negatives.

Apparently I was wrong.