I was the same dilemma 1 year ago with my EM5 mk 3 with 6 M43 lenses.
I was choosing between upgrading to a new Sony System or the OM-1.
In the end, I go for the OM-1 but I waited for the birthday month for Olympus Club and get a 10% discount.
I finance 40% of it by selling my EM5 mk3 (I got a good price from a nice gentlemen as mine is in a mint condition).
The OM-1 is very good if you are into birds in flight and also if you want to continue to do things with their Live Composite.
With the Pro Capture, I can now capture birds taking off flight and landing too. It is really fun.
Also the Live ND is great when you want to travel without ND filters.
Plus M43 have access to lenses like the 100-400 or 100-450 which gives me a more budgetary solution to reach 800-1000mm for shooting smaller wildlife and birds due to the 2x multiplier on focal length. Full frame lenses to reach above 800mm cost a bomb a weigh a ton!
Having said all the good things above, my son is using a Sony A7 iv, which I have borrowed on some of my trips where I take landscape.
Generally, the dynamic range is better, which will come in handy when you take photos during Golden and Blue hours.
I choose the OM-1 as my main camera as I do not take Blue and Golden hours photos that often and 20mp is really enough for my A3 size photos in photobook and social media. I am sure there are other benefits but I do not use Full Frame that often so I cannot comment.
At the end of the day, what you choose it depends on your usage.
I was choosing between upgrading to a new Sony System or the OM-1.
In the end, I go for the OM-1 but I waited for the birthday month for Olympus Club and get a 10% discount.
I finance 40% of it by selling my EM5 mk3 (I got a good price from a nice gentlemen as mine is in a mint condition).
The OM-1 is very good if you are into birds in flight and also if you want to continue to do things with their Live Composite.
With the Pro Capture, I can now capture birds taking off flight and landing too. It is really fun.
Also the Live ND is great when you want to travel without ND filters.
Plus M43 have access to lenses like the 100-400 or 100-450 which gives me a more budgetary solution to reach 800-1000mm for shooting smaller wildlife and birds due to the 2x multiplier on focal length. Full frame lenses to reach above 800mm cost a bomb a weigh a ton!

Having said all the good things above, my son is using a Sony A7 iv, which I have borrowed on some of my trips where I take landscape.
Generally, the dynamic range is better, which will come in handy when you take photos during Golden and Blue hours.
I choose the OM-1 as my main camera as I do not take Blue and Golden hours photos that often and 20mp is really enough for my A3 size photos in photobook and social media. I am sure there are other benefits but I do not use Full Frame that often so I cannot comment.
At the end of the day, what you choose it depends on your usage.
Abit of my story.
I started with Canon SLR, then Panny mirrorless. Lighter, smaller, cheaper, smaller accessories, smaller bag.
During a holiday trip, I smashed the camera on a concrete floor, by accident. The cam strap entangled with something, I pulled with force. When it untangled, it slipped my palm and smashed to the floor. Heartache. Yes, very hard hit, but the frame and lens had no visible crack. The cam refuse to boot and kept resetting. It was at night. I didn't have a spare cam. Doomed. The next morning, I checked the cam, everything back to normal. I went on to use it for another 3 years. No problem at all. My highest respect for Panasonic.
Then I switched to Oly EM5 Mk 2, due to the lens are compatible. But I still bought 2.8 pro lens. Served my well for 5 years b4 mold grew inside the lens and cam. Sent both for cleaning by Oly service centre, not cheap, but I convinced myself, they're still good and still have net book value. Result was good. But they didn't advise and I forgot to ask for the internal battery to be replaced. So, the date need to be set every time I change the battery. Learnt my lesson, I bought a dry cabinet. As memory card and SSD prices dropped, I changed the photo taking resolution to the highest and raw file. Hardly play with the raw files, one, no time, two, seldom got complaint with the jpg version.
Niece, arrowed to be a photographer, first-timer, borrowed my cam once for her uni beauty pageant contest. She returned my cam in good condition and complimented her photos were better than her peers.
Now 8 years on, my Oly cam is still good. However, the video is still HD, bad when viewing on monitor or TV set. Now, I'm in the market looking for upgrade. To my horror discovery, left only Alan Photo carrying the brand. The world has moved on to Sony and move back to Canon.
I still prefer mirrorless because my audience don't look at my photos larger than their hp screen, not more than 1 sec. Very sad norm, nowadays. I also don't develop hardcopy or do business. Only me, an uncle who would look at them on a 27" 4K monitor for the longest time. So, APSC, FF are meaningless.
With my budget, I would have chosen OM5, around $1.6k (body). OM1 is a dream but too expensive, "ideally" around 2.7k (body only). Reading about the uncertain future of mirrorless especially the development of Oly's small service ctr, poor sales worldwide, I bravely looked at other brands. A cheap body (e.g. Sony 6700) plus a good lens (e.g. Sony G Master), $5k, before a external flash light, remote control, spare battery, polariser. That's a huge investment, since I no longer take family photos or in SG, but just for travelling, which offers better and faster control, less distortion than a handphone, e.g. people on both side look wider, buildings are stretched when cam angle tilted.
Suddenly, Oly OM1 doesn't looks expensive any more. However, price at Alan is 3k+. SG onlines prices are more competitive, 2.7k, but I don't know the sellers. My upcoming trip is to Japan. A check in Japan big names like Bic Camera and Yodobashi, their online prices are cheaper. Rakuten is even cheaper, but it is pure online. Bundle with pro II lens (my current is pro I), is like getting the lens at 50-60% discount tha buying the lens along. So, it makes economic sense to get one at the beginning of my my trip, bcos S$9=JPY1,000 and tax-free. However, there are plenty of drawbacks. I don't have time to learn prior to the trip. The warranty after I left the country is as good as gone.
Looking at my situation, either I take a painful switch now, with some times to climb the learning curve of using a new camera and brand, or grab the upfront convenience and advantage to stay put with Oly and buy in Japan but prolong my dependency at the mercy of the uncertain Japan Industrial Partners (JIP)'s strategy for OM Digital Solutions (OMDS). (I dreamed of getting a zoom lens. With new and old Oly cams, this could be realised, I hate switching lens.) It is assumed my current lens can last a few more years, if staying put. Getting a new Oly pro lens in Japan is like 75% discount from says Sony G Master, but granted, Sony G Master lens has innovation like aperture (speed?) setting ring.
May I seek member's advice since I'm perfectly at this cross road now.