GS-15s aren't particularly common (fedscope data says 3.1% of federal employees are GS-15) and overwhelmingly they are supervisory positions (or more commonly supervisor-of-supervisors). Even if you do find a non-supervisor GS-15, the federal government mostly contracts development, so it's exceptionally rare to find a federal engineering position that does something other than contract oversight (generically called acquisition). I'm not saying those positions don't exist, they are just rare, and typically reserved for promotions from within the agency or federal government. Also, since GS-15 is so senior in the federal government, even if the position is technical it will start to be political.
Pay is pretty low for new grads and experienced engineers, but decent when you're in the ~3-8 year range of experience. Most agencies are hiring new grads at GS-7 or GS-9 (in SF that's $61k to $75k to start) which is a joke. So even if you can get on at a federal agency as a GS-14 or GS-15 level, you'll have an exceptionally hard time hiring new engineers. Pay at the high end, especially in HCOL areas, very quickly hits the congressional limit and makes it pointless to seek promotion (Look at the SF are GS-14 table: https://www.opm.gov/policy-data-oversight/pay-leave/salaries... ). You can theorhetically bonus up to 20-25%, and I've heard that some intelligence agencies just do that across the board, but most agencies are probably giving a 2-3% bonuses each year.
This is a sad and interesting factoid: Long ago congress passed a law saying they need to track federal vs private sector pay and adjust the pay to federal employees to match that. Of course, there is an exception that allows the president to disband this requirement in case of national emergency. So now every year the president literally writes a letter declaring a national emergency and manually decides what the pay increase will be, which is always significantly lower than what is recommended. Here are the national emergency letters from 2023 and 2022, but they have been issued by every president for decades now:
2023: https://www.whitehouse.gov/briefing-room/presidential-action... 2022: https://www.whitehouse.gov/briefing-room/statements-releases...
Benefits are good, but not amazing. Yes, there is a pension and a wide variety of health insurance options, but they're mediocre compared to what a lot of high-paying tech jobs offer. TSP is basically vanguard with lower fees and a 4% match. You can look at the healthcare options here: https://www.opm.gov/healthcare-insurance/healthcare/plan-inf...
The 40-hour work week it completely true. If you're trying to work overtime, you'll actively be discouraged and told to stop it and go home. There is generally support for telework, but it's very political, and who knows if it will get killed in the future.
Lots of agencies, especially the agencies that need a lot of technical work, will require you get a security clearance and that comes with it's own considerations. Drug use and foreign contacts are a big problem and makes recruiting even more difficult.
It's possible to find a job at some agencies working on stuff that feels very rewarding and very important, but you will always constantly feel like the bureaucracy is constantly fighting you.