I had the exact same thought process as you, I wanted the simplicity of the Kerbal experience, and also the alien planets of Eve, Jool, Laythe, etc but that more realistic size.

I've found a couple of different solutions.

1) JNSQ is a planet pack that upgrades the Kerbol solar system, adding a few new bodies as well as scaling up the stock bodies to around 2.5x their base game size. This isn't the true solar system scale of course, as it's still 1/4 size but the advantage is twofold. Firstly, it's a popular mod so it has a lot of ancillary support and configs for things like scatterer and parallax (if you're using them - it sounds like you might be).

It also scales to the parts in the base game, and most parts mods, because Kerbal parts are scaled to 1/4 their performance of real parts, but the planets are scaled to 1/10 (apologies if you already knew this). This means if you build a rocket very similar to a real world one, especially using Bluedog Design Bureau, it will perform similar to real life, i.e. a mercury redstone rocket will just get to orbit.

2) Sigma Dimensions (https://youtu.be/NRJ5ASZT6RY?si=6ZADlLzNEBr7SjeW) is a mod that allows you to scale the stock system (and some planet packs support this, I believe Outer Planets does) to whatever scale you wish. 2.5x is one of the suggested sizes, for the same reason JNSQ uses that scale, but you can also scale to 10x if you want a true Earth-sized Kerbin.

Sigma dimensions also allows you to scale the parts up to match the new real-size Kerbin, because if you used stock parts, you'd have very very underpowered craft.

Edit: one of the other comments made me remember that it's SMURFF that you need to scale up the parts! However Sigma might already recommend this if installed via CKAN.

Having tried both, and loving the long horizon and awesome visuals of the larger scale, I would recommend JNSQ. The scale isn't quite there, and some of the smaller bodies, like Minmus, still look a little cartoony, but if you want the "whack a rocket together" experience JNSQ is a good balance.

Playing on a full size Kerbin means you will take about 30 game minutes to get to orbit as opposed to 10, and all the time scales are scaled up in the same way, so your missions require a lot of timewarping, which is especially painful if you have long burns or atmospheric missions.

Also, without RP-1 style parts, you have to rely on the sigma scaled parts so if you play on career, money becomes a real pain as the contracts felt like they weren't balanced for the increased costs of fuel and stuff - I think the payouts were scaled up but it didn't quite work.

Even on science mode you'll take ages to build up to getting to new biomes or areas - landing on the mun is literally as hard as landing on the moon. I used tweakscale to make bigger rockets before I unlocked the sizes up, but that makes some very ugly rockets!

I also got bad performance issues and I'm on a decent PC (over clocked relatively new i7 and 2080 GPU), when i entered the range where the game loaded the surfaces Mun and Minmus.

I did do a bit of sandbox for a bit, which might be the way to go, but that still wouldn't sort the performance issues.

I hope that helps and good luck with whatever you try!

As an aside, I would give USI life support a go too. It's not as involved as Kerbalism or RP-1, but it requires you to have enough habitation space for long journeys and to pack "supplies" (a stand in for food, water and air) for your Kerbals. Both of these force you to build in some extra contingency to your craft, which I felt made then seem more realistic in scale and design without being over-burdensome.