>buying quake when you can easily find pak0.pak and pak1.pak online
That aside, similar to Doom you have to keep moving all the time. If you just keep legging it at top speed you can stay away from Shalrath projectiles, in crowded rooms you can also maneuver so there is an enemy between you and the projectile, which gets rid of the projectile, hurts an enemy and may trigger infighting. You don't really need to bother with Rocket Jumping, it was/is used for getting around fast in multiplayer or speedrunning. Few mappers will hide secrets behind Rocket Jumps and nothing in the original maps require it either. Same goes for bunnyhopping, I use it but in vanilla Quake it's nowhere near as effective for gaining speed as it is in QW. You can only really use it in large relatively flat rooms are corridors unless you git gud anyway.
Other tricky enemy is Shamblers, the three options are
1) Have LG/SNG, spam it to death in 3 seconds and move on.
2) Fire from near cover, ducking when it goes to fire lightning at you.
3) Get in its face to bait melee attacks and keep firing at it while dodging them, you likely take a few lightning hits this way but fewer than you would trying to fight at a distance without cover.
Besides that play good maps and appreciate that Quakespasm isn't a piece of shit like GZDoom.