A properly configured portal custom experience can still solo against bots and behaves properly like backfill allowing players to join or leave.
I have done the work for you. Here are a dozen+ different PVPVE experiences which allow you to solo start against bots. Host with a password if you want PVE only.
Play PvE or PvP: Solo Start with Bots | EA Forums - 12909480
Casual conquest is a hit because it has all the vehicles, fast respawn, and allows you to practice and PVE against bots that behave and use vehicles in a mostly natural way (as best as is allowed at this time), allows you to start solo with 55 bots, and maintains a minimum of 16 bots even when maxed out with players.
Otherwise, if you want to do it yourself, this is how:
Create a Portal Custom experience. Edit the map spatial file to customize the spawns, objectives, add vehicles, and add AI spawners. Then create rules which tell the spawner to fill the experience with bots. The AI spawners with a rule to trigger them get around the bot backfill minimum player limit by forcing the server to be portal custom which allows EA to control XP in whatever way they wish in order to stay happy.