The line cannot be held without US, even if it goes Zelenskyy way and EU send 110,000 troop to tie in the border with the existing Ukrainian guard. Russia is going to try again and break that line unless NATO membership was granted, but if Trump or MAGA is still in power, then effectively even NATO membership is probably pointless.
What Ukraine lack is 2 things. Available Manpower and Available Equipment.
Manpower is easy, once this war ends its active stage, whoever in charge of Ukraine will most likely enact conscription act, that will expand both active and reserve manpower. The will need to double their available manpower and have a reserve corps modelled after the Israeli. Then they also need to greatly expand their Armored Corps and Air Force, currently there are 3 armoured Brigade in Ukraine (1st, 3rd and 4th), they need at least 12, what they should do is to make these Brigade organic. Because they are going to face 2 frontlines, Capital Front (Ukrainian border with Belarus) and Southern Front (Main Donbas Line) both front would have to be heavily defended with both Armour and Air Asset. The Ukrainian will also need to expand their Air Force with 4 to 5 active Wing Command, which mean they will need about 200+ Western Fighter Jet to couple with whatever legacy Soviet Jet they were flying, Russia is very artillery heavy, the only way you can fend off Russian attack is with heavy air power, that's what the West had been doing since 1949.
For the Peacekeeping Force, the first thing you need to ask is that Peacekeeping Force have certain degree of self-containment and decision making element, otherwise even if you can get 110,000 peacekeeping force Zelenskyy Asked, it's no good if they are just going to melt away first sign of contact. EU force is great, and you don't actually need a lot, as said before, Ukraine have infantry, they have a lot of infantry, but what they need is the ability to deploy force rapidly and stemming attack before it was out of control. You are expecting the Russian to come at Ukraine again with multiple fronts and with multiple point, you got to have the capability to hit all those point before they went in too deep like they had in the first 2 months in this war, majority of the land Ukrainian ceded is from the first 2 months of engagement of this war, they are now just covering the deficit, so the importance of a Quick Reaction Force to deal with mobile threat is extremely important. Which mean you need Western Air Power more than Western boots on ground. If US decided to bug out, then you will need a Unified European Air Command to monitor Russian action 24/7. You will also need 4 to 5 EU/NATO Squadron (ideally, UK, France, Germany, Italy and/or Canada would send a squadron each) and augmenting the UAF on the frontline. Which should give them a good 10 to 12 Squadrons. Another things these Peacekeeper should take is Anti-Air capability, this, along with Air Power is what Ukraine sorely needed if there is a ceasefire and no security guarantee. It wouldn't matter if the Peacekeeping force is from EU or Japan or Korea, I doubt the latter two will participate. Maybe country like Australia or New Zealand will take part? I am not sure. Mostly this is going to be EU troop though.
What I think will happen if a ceasefire deal is signed by the end of this year. Ukraine will have 10 years or so to reconstitute its force, what they will do is again, first, get conscription in probably for both sexes, and then they will most likely be using their resource (grain, oil, gas and REM) to make deal with EU and/or other country, like South Korea in exchange for Military equipment. Expanding Ukrainian Warfighting Capability and then augmented by an Interim European Force that focus on Air and Armor, that's probably what Ukraine needs to plug the gap.
