All the Mods 3 version 5.1.

Just a note: It freezes, not crashes. If you wait a few minutes, you don’t need to restart your client.

So this isn't an "answer" per-se but this is normal. Reloading resources in modpacks since 1.11 or even 1.10 have resulted in freezing/crashing/mixed-up texturemap, etc.

I don't think it's "okay" but I also don't think there's anything that can be done on the modpack end as the issues are likely deep-rooted and quite low-priority (rebooting the client is all that's needed for the changes to have taken place and be fine).

I do appreciate taking the time to report it but for now we're not going to worry about trying to figure this out.

Ahh well yeah that's basically like what the game is doing when you first are launching minecraft, a large percentage of that long startup time is processing all the sound/texture/model resources. When you change those in-game it has to re-load allll of them again and it can't do anything else until it's done. You can even experience this pause/delay in vanilla minecraft when adding/changing texture packs but it's not as long since there are far less total resources in vanilla vs a kitchen sink pack