Awesome! I wouldn't have thought that it is possible to make ICs in a garage. Of course it requires a lot of knowledge, etc. But still, not a multi-billion dollar clean room with specialist equipment.
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However you cannot make useful digital circuits. For digital circuits, the best that you can do is to be content to only design them and buy an FPGA for implementing them, instead of attempting to manufacture a custom IC.
With the kind of digital circuits that you could make in a garage, the most complex thing that you could do would be something like a very big table or wall digital clock, made not with a single IC like today, but with a few dozen ICs.
Anything more complex than that would need far too many ICs.
Then, of course, if by "useful" you mean "commercially viable", it is indeed not going to be competitive against either TSMC or your local 500nm foundry ever.
I call such a CPU as not useful.
It can be a very useful experience to design such a CPU, but you can simulate the design in a logic simulator and you gain nothing by building it.
As a valuable computer building experience, it is more useful to use much older components than digital integrated circuits, where you can see nothing without special instruments, e.g. you can build interesting computer blocks, like adders, registers, counters etc., made with electromechanical relays or with neon glow lamps, where you can see with your eyes how they function.