There are more people than you'd expect who can talk the talk but can't code at the same level. In the majority of companies, I'd bet there aren't too many openings for engineers in such a narrow role. If you don't want to work with an 'idea person', would you want to work with an 'idea engineer'?
Multi-day interviews are likely overkill, but a full day interview is completely fair. There's no need to do 4 back-to-back algorithm whiteboarding sessions, but testing other aspects such as data modelling, general architecture, and coding style/ability are all high-signal.
Multiple days? Not a chance.
If you were paying 2x rates or real equity, ya I'd slog through a day long interview, but I bet you aren't.
What does your company have to offer that would compel me to spend 8 hours interviewing that another company doesn't?
Maybe they'll be only one company willing to pay then, maybe that will be the one with the one day interview :D
Something that's worked well for me in the past is to secure at least 1 good offer - compensation you feel comfortable with, recognizable brand, but still an offer you're willing to walk away from - and then give notice.
Then, after leaving, spend additional time (up to 4 weeks) interviewing with all the other companies you want to. Keep in mind that many companies offer a sign-on bonus, and using it to cover this in-between time is high-return, likely more so than buying a new car/vacation/entertainment system.
You can be upfront about this break in your interview loops - "I'm no longer working at company X, but I have a very competitive offer from company Y and considering several other ones." There's no guarantee you'll be able to keep all the offers you receive (many of them will explode), but you should have a steady pipeline.
Whenever you feel comfortable, you can exit the pipeline, and begin negotiation with the outstanding offers you have. Last I did this, I went onsite ~15 times among even more interview loops.
There aren't that many good companies to work for in any area (even in SV), to have 10 on sites simultaneously, and you might burn bridges for later [e.g. don't reapply before 1-2 years policy].
IMO: If there is that much interest for you, you need to charge more.
I have a hard time taking seriously a company that doesn't give me 25 days in EU. (better be with 25 sick days :D).
For me it isn't the PTO. Say I want to interview at 10 companies to see which is the best fit. That's 10 days of interviewing. If they are all 3 day interviews, that's an entire month.
I get tired of talking to people about nothing after an hour. If someone wants to bring 5 people for me to talk to about smalltalk and nothing really of substance, I won't do very well.
20 years ago, a friend who worked for a guy who would hire 10 people then after a few weeks, fire most of the ones leaving just the best. I live in an "at will" state. I guess most people just don't have the stomach for that sort of thing, so they try to make up for it with 3 days of doing nothing really relevant to the job.
Make a 1h test, NO fancy problems, just the basics. Think: summing some numbers, displaying some stuff on the console.
That doesn't substitute to the on site but it goes before to ensure a minimum level and some screening.
It's win-win for everyone. It's efficient and straightforward. On the top side, it's easier for the candidate to take 1h whenever he wants than to schedule a full phone call that will ruin half a day for both of you.
Every founder thinks their company is great. People who want jobs will happily reinforce that delusion to get the job. Some companies are better than others, but most of it is really boring stuff. I guarantee that everyone working at your company is there because they need a job.
It's not even about being attractive to juniors, seniors or mid level. It's just looking for desperate, even if you have a brand name.
Screw the rest, seriously, it's just not worth the trouble.
A couple of weeks back I saw a repo on Github. Preparing for an interview @ Google. It was a mile long. Never doing that.