Typo3 LTS9 Routing: ReUse config.YAML - yaml

Question

What's the right way to reuse config.yaml for routing?
Situation:
I have a typo3 installation with different websites. The routing-config with more than 200 lines is saved in:
\typo3conf\sites\website1\config.yaml
What the right way to use this config.yaml with all other websites and only reconfig the differences in:
\typo3conf\sites\website2-100\config.yaml
With typoscript this where easy. Is there a way to use this:
1. <INCLUDE_TYPOSCRIPT: source="FILE:\typo3conf\sites\website1\config.yaml">
2. Overide differences like "rootPageID: 2" or delete configs with ">"

Answer 1

To elaborate Jonas' hint, here a more concrete example:
Site configuration for the single site which is similar to all other and thus should be as short as possible, stored in sites/mysite/config.yaml
rootPageId: 14523
base: 'https://www.mysite.mytld'
imports:
- { resource: "EXT:mysitepackage/Configuration/Sites/defaultSiteConf.yaml" }
Content of EXT:mysitepackage/Configuration/Sites/defaultSiteConf.yaml, the default config shared for basically all sites (although exceptions are possible).
imports:
- { resource: "EXT:mysitepackage/Configuration/Sites/errorHandling.yaml" }
- { resource: "EXT:mysitepackage/Configuration/Sites/language.yaml" }
- { resource: "EXT:mysitepackage/Configuration/Sites/route.pages.yaml" }
- { resource: "EXT:mysitepackage/Configuration/Sites/route.tt_news.yaml" }
Content of EXT:mysitepackage/Configuration/Sites/language.yaml, as an example that the default config can be split into multiple files to track changes more easily
languages:
- title: Deutsch
enabled: true
base: /
typo3Language: de
locale: de_DE.UTF-8
iso-639-1: de
navigationTitle: Deutsch
hreflang: de-DE
direction: ltr
flag: de
languageId: '0'
- title: English
enabled: true
base: /en/
typo3Language: default
locale: en_GB.UTF-8
iso-639-1: en
navigationTitle: English
hreflang: en-GB
direction: ltr
fallbackType: fallback
fallbacks: '0'
flag: gb
languageId: '1'

Answer 2

If you use TYPO3v9, you can use imports:.
Documentation:
https://docs.typo3.org/m/typo3/reference-coreapi/master/en-us/ApiOverview/Yaml/Index.html
From TYPOv10 on they can be relative, too:
Changelog:
https://docs.typo3.org/c/typo3/cms-core/master/en-us/Changelog/10.1/Feature-88742-ImportYamlFilesRelativeToTheCurrentYamlFile.html