Sorry, I missed this post earlier.
F-Droid is an alternative, but it is not perfect, indeed (I would argue Play Store also misses things F-Droid does have).
If all you need is a viewer, then LibreOffice Viewer | F-Droid - Free and Open Source Android App Repository should be suffice. If you need to edit Office documents, I’m not sure it is a good idea to do that on a smartphone but you are right that there is no full office suite available on F-Droid.
Other than Purism and /e/ I’m waiting for the release of:
https://www.pine64.org/pinephone/
I already use some of their products and I find the project very interesting.
I have already done a post on the other thread and but I repeat my point of view here:
I’m really looking for a phone done FOR the people:
for the people who work and create it, the people who use it.
I think should be considered as a whole project pattern.
Why is so difficult to find a product like this?
Fairphone is unique in the vision:
I’d rather have half the guarantee and a chance to own my phone, from the root password up (you heard, “If you don’t have a root password, someone else does.”).
I’m sure there will be a product that will fulfill all these wishes.
I would like it to be FP3 .
Where is the Open Fairphone OS for FP3? I’m still waiting for it before ordering.
Don’t hold your breath just now, it could take a while …
Does anybody know, whether VoLTE (Voice over LTE) is/was working with Fairphone Open OS (or e.g. Lineage OS, Ubuntu) for the “old” FP2?
And whether it is expected to be working on e.g. Fairphone Open OS for the “new” FP3?
This is because VoLTE is not implemented in the AOSP and that should be done by phone manufacturers or someone else. Usually with Lineage OS VoLTE is not working.
Without VoLTE working on the FP3 with an Open OS it is not interesting for me. But I would be sorry about that.
I did not use it, but there was some discussion on this topic, like this thread:
Maybe this will help You?
Hello !
Any news for FPOpen on FP3 ?
Or maybe LineageOS ?
It’s so sad to only have a stock Android on this phone…
I’m pretty sure there won’t be any news about Open OS any time soon. Ask again in a few months.
As paulakreuzer says, it will take a while… Any developer first of all needs the device for which he wants to create an Open OS. So it’s quite premature to seek for it by now. But, as many others here, I’m sure in a few months the time has come.
As my username reveals, I have any reason to look forward to an Open OS, too… 
I would personally be willing to try and port twrp to my FP3. However to do so I need stock recovery ROM. Where can I find it?
I didn’t actually ready well before buying (my fault) and assumed FP3 would come with an Open OS, I was so disappointed it didn’t… Me too, I hope they’ll work on it!!
I wish it wasn’t based on Android. I happily use Ubuntu Touch on my FP2. 
Why does the devs not release the source code of the current OS? So the development of lineage maybe gets faster.
Well, you could ask them to release the source, and they will have to comply with GPL.
I think you can ask guidance to @z3ntu, who maintains TWRP for the FP2 (if I’m not mistaken).
It’s not much about the source code itself (most of Android is open source) but the “binary blobs” (drivers for hardware such as modem) that are closed source, from the hardware manufacturer.
I’m currently waiting for my FP3, and hoping there’ll be a de-Googled OS to go with it when it turns up in November.
Do we have a list of proprietary device drivers that we should be looking at finding alternatives to/integrating if one were to hypothetically build an Android derivative for it?
Please do some reading in this thread.
And I guess you will have enough time to do so until a google-free OS turns up.
As was already posted: The phone just entered the market, so developers barely could lay their hands on them to start working on the OS. As they most likely will not be paid to do so, they will do it in their spare-time. Give them at least a few months. I am confident, there will be an alternative OS, but I would not expect it to happen befor easter 2020 (withsun 2020 seems even more likely to me).
(Edit: just corrected a misspelling.)
And in case Owlbear meant the FP Open OS provided by Fairphone and not a community alternative, I’m pretty sure that won’t be there by the time the phone arrives either.
I suppose what I’m really looking for here are resources for people who would be willing and able to contribute to the development of alternative OSes for FP3. That’s probably a bit beyond the scope of this forum, but I’ll see if there’s anything happening over in the Ubuntu Touch and LineageOS communities.
Apologies, I didn’t mean my ill-considered post to come across as someone not who’d bothered reading - I know my hope is rather over-optimistic, but wanted to add my voice to those with an interest in an open OS.