fghj753 (2022-09-19 21:06:28+00:00): Warning: very long message. You can just copy this message to Word to make reading it more bearable.

Tere,

First off, let's start with a brief overview of history. When i became OSM mapper, there was an active building import by SviMik that was focusing on adding new buildings from Maa-amet (MA from now on). SviMik had a policy, that to avoid possible editing conflict (conflict as overwriting someone's work without their consent), his bot would only either add new buildings or update addresses on buildings last edited by selected users/bots (mostly large volume users such as Verbatium, xybot and juhanjuku). At same time buildings added by his MA imports had very high data quality. Because of trying to not interfere with possible future imports, I'm still not really proficient at mapping buildings.

Collection of some resources regarding to SviMik's import.
SviMik's OSM website that was used to validate the data before bot imported it - http://osm.svimik.com/?lang=en
Import notice/documentation on wiki - https://wiki.openstreetmap.org/wiki/Maa-amet#Import
Talk-ee mailing list on Svimik import - https://groups.google.com/g/openstreetmap-ee/c/HnD85p3Wg9s (there are few other threads related to imports there, but it's not worth linking them here)
SviMik_import - https://www.openstreetmap.org/user/SviMik_import

Importing new buildings as outlines is not really difficult per se, just download ETAK data shapefile from MA and upload them with Josm. Problem here is the OSM's expectations for quality. For example, the most obvious step is adding tags that are suitable for OSM object and not uploading buildings that are already added to OSM. However, there are more nuances which reveal themselves after you have decided to start this process. In Estonian context, MA's geometry data is sometimes too high quality for modern OSM expectations (e.g having 40+ nodes per circular way while OSM standard is 20) or has minor artefacts caused by technical limitations of their internal software (complex issue, but it results in too many nodes on a straight line). 

Those were just issues i had with geometry import. Another dimension is getting the address data right. My import focused only on geometry, because tags were already updated by SviMik's import in 2019. However, i noticed that address field in the dataset was just single text field - it would be really difficult to try to generate address tags from this field. More reliable source might be MA's ADS (aadressiandmed), but since that's not downloadable, just an API, then i believe using it would quickly become bottleneck for the import process. SviMik's source code for import bots is not public, mine is indeed on github repo linked above.
In future I personally would like to also incorporate data from Ehitisregister (EHR), because from there we could get data for building level, building materials, age etc for hundreds of thousands of buildings, but sadly that's unresearched topic, meaning OSM community might not have knowledge on how to obtain and process EHR’s data or if it’s even legal (by having compatible license). MA and EHR aren't directly linked either, so it would require lot of additional work to get buildings in two datasets to match each other.

My import on wiki - https://wiki.openstreetmap.org/wiki/Maa-amet_building_geometry_update
ETAK dataset on MA server - https://geoportaal.maaamet.ee/est/Ruumiandmed/Eesti-topograafia-andmekogu/Laadi-ETAK-andmed-alla-p609.html
EHR example data https://livekluster.ehr.ee/ui/ehr/v1/building/120772119

Replying to qqqqqqqqqqqqqqqqqqqq's questions:
0.1) There are various wiki pages regarding to imports and their documentation. Almost all known imports are documented on https://wiki.openstreetmap.org/wiki/Import/Catalogue I think all known large-scale organised Estonian imports are featured on that table, just use ctrl+F to find them. If either of you is going to conduct an import, please start from https://wiki.openstreetmap.org/wiki/Import/Guidelines and AECoC for generic guidelines on topic.
0.2) MA-geom import's main principe was to refrain from touching nodes that hadn't been updated for at least decade. My initial plan back in 2021 was to run second phase of import in spring 2024 (to update SviMik's first import) and then third run around 2028..2030. I have no idea if SviMik has any plans with his import.
1) Yes, that's the script. Can't give any guarantees if it's going to work.
2.0) Regarding other importable building properties, refer to paragraph on EHR.
2.1) Road refs are relatively easy thing to import. You need to keep in mind two main caveats: 
2.1.1) Roads on ground (and in OSM) usually follow logical path while road registry (and therefore ref-numbering) follow parcel and local gov boundaries, meaning that you will need to pay extra attention to matching lot of parallel and/or intersecting roads and splitting OSM ways accordingly. 
2.1.2) OSM has route relations, so you'd need to put in extra effort to your import script to make sure you don't break them while splitting ways.
2.1) (continued) Past summer I was travelling between Tallinn-Pärnu and discovered there were discrepancies not just on-the-ground and in road registry, but even same road's different ends had different names. Best known example could be Pärnu maantee, which is called that way in Tallinn, Tallinna maantee in Pärnu, but Tallinn-Pärnu-Ikla road in official registry. Sample changeset/region: https://www.openstreetmap.org/changeset/123379963 (this one was done by hand.)
2.2) As for finding other potentially interesting things to add, take a look at different applications and map layers offered by MA: https://geoportaal.maaamet.ee/est/Kaardirakendused-p2.html
Back in 2010 there was navigation aids' import. That might have some interesting value, but problem is that due to legal reasons MA is not really keen to distribute machine-readable data about them. Sea charts have information about shipwrecks, this information can be connected with Vrakiregister to add shipwrecks to OSM.

P.S I was told this comment should be a wiki page. Problem is that I'm replying to your discussion too directly, so converting it to wiki page will require lot of editing.