I tend to do better work on other peoples blasters, and leave some effort and concern out of mine because I can always fix it, and there is an experiment or unknown quantity/part of some sort in every blaster that is mine!
M120 spring is never going to end well in a nylon box! and 60% cylinder? why? Clearly this customer wasn’t “right”, and then it came to you? 
PTFE tape is a pet hate of mine… it isn’t for sealing, and in the rare scenario where it can help it is used wrong and in such vast quantities that it doesn’t work for long anyway! The CYMA 460 motor is not worth putting a metal pinion on. A wells or SLR motor is a big upgrade, and though not great motors, says alot about the cyma 460!
If it has had an m120 spring in it you may be dealing with a box that has been warped, which will have unknown reliability and may be the cause of strange semi issues, it would be hard to nail down the issue.
If you find yourself spending a lot of time trying to nail the issue, I would try to convince him to go back to nylon with good seals and mild spring. CYMAs nylon box in that for is usually pretty reliable and not bad performance.
Assemble the gearbox without the compression parts, put in the receiver and turn the gears manually while in semi with the trigger pulled, should give an idea if the semi auto mechanisms are working.
Semi auto issues can really do your head in. It is why I keep a couple of T238 Basics in stock at all times. Can get them for around $40, by them time you replace springs, trigger blocks, cutoff levers that might not be right for the gearbox, and different triggers… and spend time on multiple round trips in the gearbox… cost adds up quickly and may still not work!
If I can’t get it to semi after a few in and outs of the gearbox; I know it could take many more so it is cheaper for the customer to spend an hour of labour and a T238 Basic, and then they have reliable semi guaranteed.
As a side note, I can’t charge for work that doesn’t work out, I haven’t got it in me to do that!
For me to charge for the ETU and fitting, only works out $20 more expensive than had the semi worked from repairing the stock trigger setup. Most people are very happy for that, and that conversation itself usually results in them saying to not bother with the trigger block/semi issue and just go for the ETU, it is known results and removes a lot of variables.
Saves my time, using parts that may not work, and they get a blaster with a more sensitive trigger, auto prime, the option for binary, reliable semi and they can choose any trigger as the switch is very compatible with aftermarket triggers, where a stock trigger block is not.
Sad part is I quite like the feel of a good trigger block setup, the pull, click and reset etc feels really good when done well and working properly, and an ETU totally removes that. But most people like the shorter pull and sensitivity an ETU provides.