If there isn’t enough ammo, you’re thinking wrong.
There is infinite ammo for every single fight.
Doom Eternal requires you to re-arrange your thought process for how you analyze the fights.
The small, one-shot demons are not your enemies. They are your Ammo Packs, your Armor Packs, and your Health Packs.
Make sure to bind your Grenades, your Flamethrower and the switch between frost and frag grenades to easy-access keys.
Light little guy on fire, activate chainsaw. Armor + Ammo.
Light small pack of little guys on fire, drop frost-grenade with the right perk, switch and frag grenade them as well. Massive Armor + Health pickup for no ammo cost.
Save your ammo for the real demons. Anything Imp-level and below is walking health pack and not worth using ammunition on, some health packs are more prickly than others, though.
If you want super easy mode, get the Grenade Launcher attachment for your pump shotgun. Once you get used to it it lets you very easily disarm Mancubi, Revenants, Arachnotrons, and one-shot Caecodaemons.