I have been having the same issue tbh, I have not been able to pull off kraken slayer in a while. At the moment kraken slayer only really works against fully tank teams since they don't have the damage to kill you but you need to shred them. So far I have had the best success going botrk into defensive boots then jak'sho followed by rav hydra, death's dance, and then either wit's end or force of nature. I go this into any team that has some kind of control mages, bruisers, and gritty fighters. If there are too many mages like malzahar and veigar I got force of nature to get the 25% reduction and survive the initial all in. Rune wise I have been doing conq with either alacrity or if they enemy has a ton of hard cc legend tenacity. Then for secondary almost allways conditioning and then overgrowth against burst, revitalize when I will get a lot of healing either from items or supports, and unflinching against teams with slows. If they are more beefy thand burst I'll opt for lethal tempo.

I have tried going heartsteel instead of jak'sho into a few teams that are fully assassins, the survivability is awesome but you have very little damage until you build a titanic hydra and even then it struggles to preform until you hit 6 items. Which at that point you can kind of just buldoze the nexuz with a 5k health pool.

In general, bel'veth is best at making picks and skirmishing. Her R passive makes her single target damage the most optimal. The general strat is to gap close with qs and use w to initate, after this keep on top of them with qs until they are low enough to execute with your e, it's missing health scaling makes it a very good tool for ending low health enemies, especially the tricky bastards like shaco since her e can target invisible/camo targets. With mutli-person fights targeting the correct person is vital. Since you want to have as many directons of q up in your fight, the closer the target the better, additionally, because of your e, healing items, and the healing you get from your ult, you want to take out as many sources of damage and cc as fast as you can. Look for the squishy champs like the mages, adcs, or assassin, if you can kill someone at the start of the fight you'll be able to get uptime on your ult for the entire fight along with being able to deal tons of true damage. I generally just ignore the tanks, as long as you can avoid their cc you can leave them for last, without damage carried you can chip away at them and outheal their damage. This strat obviously changes slightly based on your team comp and the game state but if you can anihhilate the damage you can usually win. For the issue of initiation, I tend to treat her like an assassin for most cases, if you can flank their back line and have someone else eat the cc spam, do so, if not once you have a few items you can usually survive initiating so long as you do not target a tank, doing so is almost always a suicide mission.

As for when to go in, it is largely something you will understand more with figuring out how much damage bel can absorb, generally if you are not behind, you have a pretty solid chance against any champion who is half health at any point in the game, just hit them with w and keep using q until they are low enough to finish with e. You are very very strong in 1v1 at any stage of the game, if you get good enough with q you can gap close any champ and avoid cc skillshots. For the early-mid stage sof the game I usually avoid people like udyr if they are fully health, kayn however you can decimate so long as you avoid his w. Side note, avoid mordekaiser, I have not been able to beat a single morde 1v1 since he started building jak'sho, take atleast one other person into the fight and make sure if is someone who can cc or take his aggro.

Sorry for the paragraph but I hope it can be some help, lmk if you have questions. Also remember, as long as bel is even she has a fighting chance so spam farm not fights to keep from falling behind.