If you want a level of commentary that is more elevated than what I said but less than a tax attorney, I can see if someone else has anything to offer. @Carl @Hal_Al @DoninGA
What I don't know about LLCs will fill several books.
As a general principle, separate businesses should have separate taxes. This is hard for me to explain except in the breach. Suppose you are a successful architect, and you cater backyard BBQ as a side hustle, that never turns a profit. Your ability to deduct losses is limited if you never make a profit on the side hustle, but if you combine businesses, you can easily deduct all your BBQ supplies and equipment against your architect income. That's where treating dissimilar businesses as one business becomes abusive and improper.
It doesn't sound to me like your various computer services would have that problem; the income streams and expenses are similar. Some of the expenses would be the same for multiple lines of work, and keeping them under one business may actually be helpful instead of having to allocate one expense between several businesses.
It's helpful to have separate bank accounts but not required under the tax code, as long as your recordkeeping is adequate. (You can technically run all your businesses through your personal checking account although personally I would never do that for many other reasons; legally allowable is not the same as smart or prudent.)
As far as running different lines of business under one LLC, there is a legal question and a tax question, neither of which I can really answer. I can't think of any reason that you can't have one LLC and several business names (d/b/a) although that would depend on state law. On the other hand, I think if you have several businesses under one LLC, the IRS is going to want to see only one schedule C. If you filed separate schedule Cs, one would be for the LLC and the others for unincorporated sole props or other disregarded entities.
Maybe others can help.
