Is there a way to purposely get lower dungeons in roulette?

[Jeeqbit]

I am so bad at tanking but I want to practice while earning XP. I am comfortable with dungeons up until Stormblood, after that I don't know the fights too well and I have issues memorizing things nowadays.

You can do Shadowbringers and Endwalker dungeons with Duty Support. The npcs will do all the mechanics and teach you the mechanics in a place where players don't have to watch.

If there isn't, what content would you suggest for me? My Gunbreaker is level 84. Should I stick to Fates?

The level 50/60/70/80 roulette doesn't give a lot of experience, but it works. You don't need to know the mechanics very well because the sync isn't very tight.

You could also do 8-person trials and alliance raids, where there are other tanks to handle things if you die.

As you suggested yourself, you can do fates but you can also do beast tribe quests and daily hunt bills.