DIY PEX pipe installation and replacement of steel pipe

Depends on how dedicated a DIY'er you are. I have my own set of PEX tools for crimp or expansion fittings so for me it's an easy DIY, you may not want to make the investment in tools and time for a single job though. Also, for water pressure situations, PEX is often the culprit more than the solution depending on the system used. Crimped PEX fittings really cramp down the overall inside diameter, you need to upsize to a 3/4" PEX to meet the need of a 1/2" copper run. Expansion PEX systems don't suffer from that issue though.

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Not sure this solves your pressure problem,

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So then it is a plumber job since the pressure is all I’m after. The trickle of water is making washing dishes a total pain in the arse.

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Have you taken the screens off your faucet to make sure they aren’t clogged?

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Yep. Faucet is only a year old anyway.

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Gotcha, has it had low pressure since it was installed a year ago or is this a new issue?

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Low pressure before install. Gradually getting worse though.

Home inspector said pressure for the house is fine and that it’s probably the steel pipe.

after the shutoff, if you have a removable cartridge valve on the sink, check it too. We had a shower that had low pressure and I shut off the water and remove the valve cartridge. It had some screens on it that were clogged.

If possible you could also try unscrewing the shutoff valve with the water turned off at the street and see if debris has built up before the valve. maybe even turn it on real quick with a bucket somehow rigged or held to help blow anything out too.

Couple of things not mentioned

Faucets (valves) have a flow rate. They're seldom talked about but listed somewhere on the packaging. It'll be listed as GPM. And fortunately or unfortunately depending on how your looking at it.. these have been lowered several times over the years. So if your new valve has less flow than before (and the line and screens clear) this could be the culprit. You might check the model number online and see what it's rated. Then perform a check like this one... here. It's for a bath and shower but same principal. Some of the new ones are embarrassingly slow and it's really not a matter of price. It's a matter of design.

Another consideration

Instead of purchasing the equipment for crimping PEX (either major method) you could go with a Shark-Bite type coupling. And while it may indeed have that little inline piece to reduce the flow somewhat... I'm guessing it'll still deliver more than the flow rate of most new valves. They are a bit pricey; however the total price of this method will be way less than any other way (this side of borrowing the tools).

One caution: fasten the pipe well (using a fastener) so that there's no side pressure on these connections. They do not function well (meaning they'll eventually leak) with much lateral/side pressure. So while you can bend PEX on a run; always fix/attach to something solid near the fitting. Support it. Another hint: pencil mark the PEX an inch (perhaps) from the end... so you can tell how far it's into the fitting when done.

eta: USE TWO fasteners one may not be enough

I just got through doing a PEX (Uponor/expansion) re-pipe of my entire house, well...almost done, to replace the old galvanized steel pipe. There were times through the process where I was turning the main water supply on and off again since I kind of replaced it in phases to keep running water most of the time. In doing this, our kitchen sink, which had decent pressure before, all the sudden got noticeably worse (still connected at that time to galvanized pipe). Figured it would get better in a few weeks once I had replaced the pipe to PEX to the sink. But it did not. Even when connected to PEX. I cleaned all the screens and there was noticeable rust in them but it did not improve the pressure.

I ended up disconnecting the flexible supply hoses under the sink (both hot and cold) at the valves and put those ends into a bucket. With the screen and aerator removed, I took my air compressor and shot air at the faucet down into the bucket. Sure enough, chips of rust shot out and pressure was good again.

If OP has steel pipe his issue is likely mineral build up in the pipes reducing the flow. While Pex does have smaller inside diameters then copper, a 1/2 pex is still plenty for showers and faucets. 1/2 pex will deliver more water then most shower heads and faucets can deliver themselves.