I get this sort of question a lot from my clients, and in response I usually flip it back with the question would informal/casual content be off brand for their business.
I'm going to start with your latter 3 more direct questions and then end with where/how you should host the blog.
- Is it harmful to have an off-topic blog hosted on the primary site?
The question is how off-topic. To me, an off-topic blog would be like a metal fabrication shop blogging about cupcake recipes.
That would be harmful for the metal fab shop because the semantics are all over the place. This stark of a contrast would surely throw a search engine for a loop.
For a software company, posting about technical and programming subjects will do nothing but drive more traffic and increase your thought leadership within your industry. While the content might be "off-topic" and not as interesting to your customers, they might like it, because technical content is authoritative. The effect might be that it increases their confidence in working with you.
If your business was to come to me and ask "Hey Mike how can we increase our traffic?" I would recommend potentially "off-topic" content as a strategy.
- Are there SEO benefits if the tech blog attracts links?
Tons. Just because you already have a high domain rating does not mean you don't need links! A evolving, changing, living backlink profile is how a site remains relevant in the eyes of a search engine.
Attracting links is beneficial for a variety of reasons. Here's a great article about many of the direct benefits of link building.
- Are there SEO harms if those links are using keywords that aren't our primary keywords?
No, the result will simply be that your site begins ranking for more keywords.
At some point as the blog grows you may risk the content cannibalizing pages that are ranking high and driving important traffic to the business. If the one of your blog posts competes for the same keyword as salesy/business blog post, but the search intent is different, that is not cannibalization.
One person could have informational intent, the other commercial intent. In other words, in the first case the user is interested in the topic overall.
Here's one of my favorite articles on keyword cannibalization.
Where should you host this blog?
I've witnessed businesses engage in a ton of internal drama over content decisions like this. Although it wouldn't be a concern of mine, I can't say that potentially not jiving with your customer base is not a valid concern. If we were dealing with a new site fresh domain I'd be for hosting on your main blog to build up your authority and get things moving.
In your case, since we're dealing with a matured site, I would throw it on a sub-domain!
acmesoftware.com // Official Business
blog.acmesoftware.com // Your blog
No harm in that, and you'll still get the benefit of all the topical association, while also building a separate (yet connected) backlink profile.
One of my most opinionated pieces (which is broadly about my philosophy toward SEO) has been incredibly popular with my clients. I've gained 3 clients to date from it without having the "sell" for a second.
In the case that a subdomain is not possible, I would probably opt to register a .dev domain. So you'd have:
acmesoftware.com // Official business site
acmesoftware.dev // Developer journal
In cases like this, if the domain has enough authority to go the subdomain route, I often prefer it.
I like subdomain here because you're talking about starting a blog that (it sounds like) covers technical/programming topics more broadly than what relates to the businesses. With this route you don't have to worry about how certain content might affect your main sites "topical reputation", if you will.
In your question you also mentioned the idea of to hosting this on your personal site - it feels like the content you want to write is a different style, prose, tone of voice, than what your company might normally post. Other developers at your business might want to post too? A subdomain allows the blog to feel like your own separate thing.
Some additional benefits might include:
- You want to use a plain Jane design (popular in the development community)
- Your main site is huge and you don't want to worry about crawl rate/budget
- You might want to run a different stack
None of this is to say that a subdomain blog is better than a subdirectory blog. Or that hosting your blog on a subdirectory is at all bad. If your company's site had a lower domain rating...something between like 0 and ~50, a subdomain would not be a great idea - id be concerned about ranking limitations (PageRank). Like I said before, high authority sites can get away with it.