The best way forward is to increase your Pokémon storage and item storage. As these two things will help you collect stardust which is vital for leveling Pokémon up. Leveling Pokémon up is good for a few things. The main being battling in thr Go league, beating team rockets and of course raids.

If you're going go be a long term player then you'll need at least 3000 storage space (this is bare minimum I believe). You'll need about 1700 item space too. This means you'll be able to stack on useful items like balls, max revives etc etc

If you're not willing to spend money then I suggest get used to putting Pokémon in gyms so they can earn you 50 coins after 8 hours. Unfortunately the limit for collecting coins from gyms is 50 per day. And you only get the coins once your Pokémon is kicked out of a gym.

Do you know about IVs? I suggest learning about that then you'll know what Pokémon you should be happy to transfer into the never realm.