I’m not good at networking, but I found a working iptables in this thread https://dietpi.com/forum/t/tunnel-dietpi-through-openvpn/765/7 and when I compare it to yours, there are some differences:
The working config is like (I edited it to match you IPs / interfaces):
Destination Gateway Netmask Interface
Default route 10.8.1.1 128.0.0.0 tun0
Default Route 192.168.4.15 0.0.0.0 eth0
10.8.1.0 10.8.1.1 255.255.255.255 tun0
10.8.1.1 none 255.255.255.255 tun0
128.0.0.0 10.8.1.1 128.0.0.0 tun0
178.175.131.59 192.168.4.15 255.255.255.255 eth0
192.168.4.0 none 255.255.255.0 eth0
So you are missing these two?
10.8.1.0 10.8.1.1 255.255.255.255 tun0
10.8.1.1 none 255.255.255.255 tun0
And this entry in your IPtables is wrong?
10.8.1.0 none 255.255.255.0 tun0
But as I said before, IDK what this is doing, I just compared a working config with yours and somebody should have a qualified look over this!