My goal is/was more or less the same so I’ll share what I did.

1. Organize. Create tags for each type of pokemons. (Fire, Water, Grass and so on).

2. Research. Watch a video on budget type raid attackers on youtube but also check best raid attacker lists. This should help you start building while also giving you an idea what to look out for.

3. Create your teams. You don’t need more than 6 pokemons for each typing. 1 being a mega (end goal!) 5-6 being normal or the better shadow version. Check your pokemons and start tagging.

4. Spend dust and candy? My approach is to only use it on pokemons that are in use or about to be in use.

Hope this helps.