How to remove EXP progress for levels?

Open the console and type "player.setlevel <#>" where <#> represents the level you want your character to be.

Be aware this might cause some odd behavior for a bit. Some quests may have been activated when your level skyrocketed and you may still encounter their triggering events in the world after you lower your level.
It will also not update the required exp to reach the next level until you level up. I.e., if you jumped to level 10 and then reset back to 1, you would need to acquire enough exp to go from 10 to 11 before you level rather than the amount to go from 1 to 2. After leveling it should return to normal.

You could also use SkyTweak to alter experience rates and it generally plays nice with other mods. If you then use Alternate Start, you can tweak the experience rates before the game really starts.

Since you are using the SSE mod Experience, before starting your new game, open up the Experience.ini file. Here you can adjust the killing, skill, and quest experience increases.

For example, in my game I realized I was levelling too fast. So I disabled the killing experience and skill gain experience in my ini file.

[General]
bEnableKilling=0
bEnableSkillXP=0

And lastly you can also adjust how much experience you gain from quests, exploring, and clearing locations in the ini file. I had to experiment with these values till I was happy with the levelling.

And lastly, you can change the experience values mid-game also to slow down your leveling.

You can't decrease your character's current level (or, rather, the experience level), but you can decrease skill levels and the amount of experience in them.

From UESP:

AdvSkill <skill> <nn>
Advance the specified skill.
AdvSkill registers the increase and will count towards your next levelup. The number represents the experience level to add to the skill (e.g., player.advskill destruction 1 will only add 1 experience towards the next skill increase, it will not advance Destruction 1 skill level). Some skills require far more experience points than other skills, crafting skills level quickly with few points, but armor skills require much higher numbers.

And from this page:

SkillAdvance Values

[Before the 1.9 patch], the following command would set the exact experience you would have in your Smithing skill to 0, also making the Smithing level 0:
player.setav SmithingSkillAdvance 0
The following command would set the exact experience you would have in your Smithing skill to 3746, also making the Smithing level 12:
player.setav SmithingSkillAdvance 3746

These values are absolute, meaning you can readjust your skill levels to approximately reflect your lowered level.

With [after?] the 1.9 patch, you can still achieve similar effects by using the advskill command together with the setav command, but it's a much less exact process. For example, to set your skill progress for the current level to 0, you would use setav to decrease the relevant skill by one, then use advskill in decreasing amounts to slowly advance your skill progress towards the next level until the skill goes up by one. Similarly, you can very roughly figure out how much skill XP a specific action is worth by saving your game, using the skill (or casting the spell) for a specific duration or a specific number of times, then noting how much progress you've made in the skill. After that, you would reload and use the advskill command to determine how many skill points it takes to reach the same amount of progress.

I think the better (saver) choice in your situation would be to opt for any of the following circumventions:

• Loading the mod 'Experience' after the startup quests have stopped;

• Trying to find (or create) a patch to counter these effects;

• Seeing if you can edit the amount of experience points gained from those startup quests.

Here is a workaround fix for your issue; I was experiencing the same issue and I solved it using this mod.

https://www.nexusmods.com/skyrimspecialedition/mods/32357?tab=files

Use this mod to hard cap leveling so that even if your over leveled via EXP you can only get access to one levels once per day. Hope this helps anybody having the same problem.