I think the problem lies with your expectations. This is very much a co-op game first, and the single-player option, while there, comes far behind. The bots are there more as placeholders for real people than as a complete team, and they’re that way on purpose. As others said, the best you could do (if you really don’t like playing with random people) is to try to find a constant group either through slogging through Quickplay for a while and friending good partners, or going to external sources for groups (like Reddit or any of the many Discord channels).
If, despite all this, you still want to keep playing solo, you do need (at least in practice) to level up and kit out at least three other characters. Do it slowly, changing your loadout, Career and character every few levels and it’s a bit less likely to feel like a grind. As a bonus, you’ll also learn what the different characters and Careers can actually do, and you may be able to utilize them better afterwards, especially if you do end up playing with others. As for kitting out the bots, there are more in-depth guides out there, but in general three points will help: A durable Career; a sniping-capable ranged weapon (like Crossbow or Handgun); and a simple-to-use melee weapon with well-defined patterns. If you’re on PC, you could also try the Bot Improvements mod (see the Steam Workshop page about that).