You have a few things going on and I can completely empathize with your being confused. It boils down to understanding just two things regarding SEO and the answer should be clear.
The first is semantics.
In semantics, sub-domain.domain.tld is weighted right to left for meaning. This is because the entire URL is read. In your case, computersafety.namesurname.com, computer safety would weigh less semantically whereas computersafety.com would weigh much more. When the URL is read and weighted semantically, name surname will dilute the value of computer safety by half.
Keep in mind that while Google does not make direct search term matches, search terms within the domain name does matter. Semantically speaking, the domain name is one of the most important indicators as to what the site is about. Term matches to the domain name do not occur unless the search result set is weak, however, the semantics value of the terms found within a domain name is really important to the performance of the site.
Keep in mind that sub-domains are individual sites and will be linked to the parent site. The parent domain and all of the sub-domains will be clustered semantically.
Taking your example computersafety.namesurname.com, the sub-domain will indicate that the site is about computer safety. Of course. However, it may not weigh as strongly as computersafety.com in regard to search performance.
The second is topic.
It is important that a site be about one thing or complimentary things more than several unrelated things. While it is good that complimentary topics exist within a space, topics with no relationship should not. Using a sub-directory under namesurname.com would not be advised because it weakens the semantic performance of the site content since the two topics are not complimentary. I realize you are not suggesting this - I am just using it as part of the discussion. Using a sub-domain over sub-directories is far more advisable for unrelated topics so that each site content can perform semantically without being diluted.
Sub-domains versus new domains.
There is always a cost factor in registering multiple domain names. If this is not objectionable to you, then the next consideration would be How much content on each topic can your produce and continue to produce? Taking into consideration content value, it is important to be able to produce enough content that your content has value not only to users but to search engines as well. It is also important that the content grow and evolve over time to maintain freshness and relevance to an ever changing audience. If you are confident that you can produce a reasonably significant amount of content for each of your topics, then it is far better to register separate domain names for each topic in addition to your personal site. The reason for this is simple. You would be taking advantage of semantics.
However, if these are smaller topics or topics with limited growth potential, then it is likely that making them a sub-domain is better.
Back to clustering your parent site with sub-domains or new domains.
Rest assured that Google will find the relationship between your sites and cluster your sites regardless. This is a good thing. You will get credit for your work regardless. If you create separate sites, it is a good idea to indicate this. You can use an about author link to refer back to your parent (personal) site to a page that describes who you are. With sub-domains, this becomes less necessary but still a good idea. Each sub-domain will borrow value from the parent domain as well as pass value back to the parent. Using sub-domains can be a way of building value back and forth without having to specify anything. The sub-domains will borrow trust metrics from the parent domain whereas for individual domains, each perform on their own.
There is value in having separate domains too. Each will perform on their own merit without upsetting the others as easily. For example, let's say your computer safety site performs extremely well while your typing fast site is having trouble finding a strong audience. The typing fast site will not negatively effect your other sites as easily. While an under performing site will effect the others, it will not be as direct. Keep in mind that your sites are clustered regardless. This only becomes important if one sites performance is poor rather than low. Only time will tell.
For individual domains or sub-domains, if any are performing poorly, you can simply delete them to reverse the effect. However, keep in mind that a less popular site can still have significant value. Just because a site topic is not about a popular topic does not mean it has no value. For example, a site with a few visitors if done well, can enjoy excellent on-site performance metrics. By contrast, a site with a lot of visitors can have limited on-site metrics due to the topic.
In the end.
In the end, for myself anyway, it would be a decision about the content I can produce and whether or not I want to spend the money. One option is to produce the content using sub-domains for a period and see what happens. If you are successful with both topics, even if one is less popular, it may pay to then register separate domains and redirect the sub-domains to the new domains. I would try and make this determination before too many links are made to the sub-domains in order to capitalize on and preserve potential.
