There is very little FOMO in warframe. The unsealing you speak of is a rotation due to the game having over a thousand of things to unlock so farming specific weapon part when you need 5 parts would be very difficult. It's there to limit the pool. If you want something specific? That's what trade is for.
Relic system is quite fair but you do need to somewhat understand the full system to see it.
The one thing that WF does fairly annoyingly are weapon slots. You have to buy them off the market with platinum so either you buy it at 75% discount or trade prime parts to get it. A single gold part will be worth more than the 2 weapon slot but it's the one example that makes you "spend money".
I'm playing since closed beta and have thousands of hours, got my wife and son into it. I must've spent maybe £300 in total (maybe more) to show my support, but I could've easily just done trading and bought all the cosmetics I wanted that way too.
WF makes you spend money on things you want to, not need to. No bullshit paid for battle pass or battle pass paid for skips etc.