Good one Les. I used to employ a switching up style like that in the early days. going up 3 reps from an initial range, tner adding a fixed amount to things and going back to the lower number.
Lif, you don't necessarily need more active downtime. If you're training hard on the weights, eating right, and adding the prescribed Cardio in, you'll be fine. Adding a lot of walking in, could, depending on duration, type of terrain, speed etc etc, be counterproductive, or hinder change, so don't think that's necessarily a route to go down, it may or may not be.
You're in a cycle of change regards training and diet, you just need to be patient, go easy on yourself, remember you're human, and just keep learning, and doing your best to change anything that currently isn't quite right and might require it.
Yes, that inconsistent fluctuation in weights your Gym has, isn't ideal, and they should be doing better to make sure weights go up in standardised increments. Any decent manufacturer would manufacture them that way.
Most dumbbells go up in either 2kg, (4.4lb), or 2.5kg, (5.5lb), increments, until you get perhaps to something like 30kg, 66lb), then they go up in 5kg, (11lb), increments perhaps after that.
You seem to be doing a great job and you should be proud of yourself. You're only 20, so you're still plenty young, learning, and adapting. It obviously will tkae time, but you will get there.
Just keep smiling, and no getting frustrated or beating yourself up, and keep focusing on where you want to be. This is not just for the soon to be, but also for the rest of your life, so think of it as a Marathon not a sprint

.