Is it possible that these low quality portfolio-pages hurt/ affect the visiblity of all my webpages, also the services pages, because Google considers a lot of my pages as "low quality"?
Yes, there is a risk of getting punishment for the contradiction of Webmaster Guidelines:
Creating pages with little or no original content
I recommend that you base your work on the content on the Search Quality Raters Guideline of Google:
Here are the most important factors to consider when selecting an overall Page Quality rating:
● The Purpose of the Page
● Expertise, Authoritativeness, Trustworthiness: This is an important quality characteristic. Use your research on the additional factors below to inform your rating.
● Main Content Quality and Amount: The rating should be based on the landing page of the task URL.
● Website Information/information about who is responsible for the Main Content: Find information about the website as well as the creator of the Main Content.
●Website Reputation/reputation about who is responsible for the main content: Links to help with reputation research will be provided.
What makes a High quality page? A High quality page should have a
beneficial purpose and achieve that purpose well. In addition, High
quality pages have the following characteristics: ● High level of
Expertise, Authoritativeness, and Trustworthiness (E-A-T). ● A
satisfying amount of high quality Main Content, including a
descriptive or helpful title. ● Satisfying website information and/or
information about who is responsible for the website. If the page is
primarily for shopping or includes financial transactions, then it
should have satisfying customer service information. ● Positive
website reputation for a website that is responsible for the Main
Content on the page. Positive reputation of the creator of the main
content, if different from that of the website.
And also on the More guidance on building high-quality sites of the Official Google Webmaster Central Blog:
if you want to step into Google's mindset, the questions below provide some guidance on how we've been looking at the issue:
Would you trust the information presented in this article?
Is this article written by an expert or enthusiast who knows the topic well, or is it more shallow in nature?
Does the site have duplicate, overlapping, or redundant articles on the same or similar topics with slightly different keyword variations?
Would you be comfortable giving your credit card information to this site?
Does this article have spelling, stylistic, or factual errors?
Are the topics driven by genuine interests of readers of the site, or does the site generate content by attempting to guess what might rank well in search engines?
Does the article provide original content or information, original reporting, original research, or original analysis?
Does the page provide substantial value when compared to other pages in search results?
How much quality control is done on content?
Does the article describe both sides of a story?
Is the site a recognized authority on its topic?
Is the content mass-produced by or outsourced to a large number of creators, or spread across a large network of sites, so that individual pages or sites don’t get as much attention or care?
Was the article edited well, or does it appear sloppy or hastily produced?
For a health related query, would you trust information from this site?
Would you recognize this site as an authoritative source when mentioned by name?
Does this article provide a complete or comprehensive description of the topic?
Does this article contain insightful analysis or interesting information that is beyond obvious?
Is this the sort of page you’d want to bookmark, share with a friend, or recommend?
Does this article have an excessive amount of ads that distract from or interfere with the main content?
Would you expect to see this article in a printed magazine, encyclopedia or book?
Are the articles short, unsubstantial, or otherwise lacking in helpful specifics?
Are the pages produced with great care and attention to detail vs. less attention to detail?
Would users complain when they see pages from this site?