If you can justify the expense of upgrading, the Canon RF 100-500 has been working spectacularly for me.

I recently upgraded from the EF 100-400 II mounted on a 7D Mk. II, to the RF 100-500 on an R5 and the results have been night and day - the RF 100-500 on the R5 body has been yielding some quite spectacular results.

If it's in good condition you can probably get $800-1000 for your EF 100-400 II on the used market, which certainly takes some of the edge off the upgrade cost, but yeah it's still pricey.