I can get the meaning the you added routing and multinet-NAT on Side A. But still need to make the traffic of Side B be able to forward to Side A. For IPsec VPN, we need to fill remote subnet in configuration. Only the traffic that desination is remote subnet will go to VPN tunnel then received by Side A. Other traffic will go to the ISP of Side B.

IPsec VPN is not based on routing. So it will not work even if add static routing. PPTP/L2TP is based on routing. Maybe you can try. As I know, the WAN of the router can be chosen as L2TP/PPTP as well, maybe it can achieve that access the internet through VPN, but it doesn't support encryption and need to set secondary IP as dynamic IP or static IP. I think because WAN L2TP/PPTP is used for getting IP address only.