At least for me, a major issue turned out to the abstractions (like wg-quick, ufw, network-manager, openwrt) which on one hand obscure the inner workings but at the same time are limited enough that at some point you will be forced to dig deeper. They also don't always play nicely together wrt routes, routing polices and *tables rules.

I wasted so much time (probably weeks, added together) in my younger days banging my head against the wall with Linux networking. Throwing out other high-level tools/frameworks and starting "from scratch" on CentOS/Rocky/Debian with only systemd-networkd and nftables has now made me mostly comfortable and I finally feel in control of the networking stack on my Linux boxes. I say "mostly" because, well, Docker and CNI...

Oh, and see my sibling comment.