Can someone recommend similar boomer-shooter inspired games that capture the Quake 1 or Serious Sam feel (And ideally the aesthetic)? I have my eye on HROT, but that's still in development.

I tried Hedon but that game was 80% backtracking.
From what I hear Project Warlock is not so great and only has the aesthetic.
Amid Evil's font turned out to be illegible for me and the demo didn't grip me.
Cruelty Squad just looks like a glorified shitpost so I'm not sure I want to suffer through that.

I've also heard of Ultrakill and Ion Fury but I've yet to try those.

I'd kill for a spiritual successor to Unreal 1 or Clive Barker's Undying but seems like most people prefer Quake and Doom.

HROT is fun. Feels more like Chasm The Rift than Quake but in all the good ways.
HEDON is more a game about exploration mixed with classic fps shooting. I enjoyed it and hope the upcoming one Blood Rites will find it's way to GOG too.
Project Warlock is more classic fps action like Wolfenstein 3D but with more weapons, some spells and a perk/skill point upgrade system. It's a fun game too.
The font in AMID EVIL can be changed to something more readable in the options. This game is more in the style of something like Heretic. Another one that's a lot of fun.
Haven't played Cruelty Squad but the videos I watched about it showed me that it's not a game for me. I was actually getting a headache from looking at that games graphic style for too long. The game might be fun but since I haven't played it myself I can't confirm that.

Ion Fury is the second best Build engine fps game after BLOOD and one of the best fps games available on GOG. Period.

Once it's fully released ULTRAKILL will probably be New Bloods best fps game. It's tremendously entertaining and I can't wait for it to be finally fully released and available here on GOG.

I also highly recommend Viscerafest (challenging but in all the good ways), Graven (Want more of that Hexen II style hub level design?) and WRATH: Aeon of Ruin (feels very Quake with a few twists here and there).

The only game I can think of that hasn't yet been mentioned is Prodeus, which is fantastic, but still in early access. It supports user created content, and so far there are tons of user made levels for it.

Oh, huh. I should try the demo again, I might have struggled to read the options because of the font. A friend recommended it to me, but the game looks too painful to even consider it.

There's also STRAFE - Anyone play that?
And yes, I know I can check reviews, but I find that I can read a hundred reviews and still get a different experience.

This option might only be available in the full version since it was added with one of the patches that followed after the full release. I have STRAFE and I played it a couple times. I enjoyed the game but it's definitely not on the same level of fun you would get from something like Ion Fury, WRATH, Viscerafest or ULTRAKILL. The most fun I had in STRAFE was messing around with the different mutators that you can unlock and also combine (unlimited combo unlocks after you beat the game 4 times). These are able to change the game quite a bit (e.g. invisible enemies + everyone just has a rocket launcher + instagib).

I would strongly recommend checking out Civvie11 on Youtube. He covers a lot of classic and retro-style modern FPS titles. He's covered everything that's been talked about here and more besides.

I have a lot of retro FPS titles on my wishlist waiting for them to exit Early Access (I don't purchase unfinished titles). To add to the list of potentially good titles, check out Gloomwood (a Thief-like title with guns) and Dread Templar (mindless fun with shotguns and kitanas).